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Review

Development of Thermoelectric Half-Heusler Alloys over the Past 25 Years

by
Gerda Rogl
* and
Peter Franz Rogl
Institute of Materials Chemistry, University of Vienna, Währingerstraße 42, A-1090 Wien, Austria
*
Author to whom correspondence should be addressed.
Crystals 2023, 13(7), 1152; https://doi.org/10.3390/cryst13071152
Submission received: 13 June 2023 / Revised: 17 July 2023 / Accepted: 19 July 2023 / Published: 24 July 2023
(This article belongs to the Special Issue Advances in Thermoelectric Materials 2023-2024)
Browse Figures
Versions Notes

Abstract

:
Half-Heusler alloys are among the most promising thermoelectric materials. In the present review, thermoelectric properties (at 300 K and 800 K) of more than 1100 compositions from more than 220 publications between 1998 and 2023 were collected and evaluated. The dependence of the peak figure of merit, ZTmax, of p- and n-type half-Heusler alloys on the publishing year and the peak temperature is displayed in several figures. Furthermore, plots of ZT vs. the electrical resistivity, the Seebeck coefficient and the thermal conductivity at 300 K and 800 K are shown and discussed. Especially thermal conductivity vs. power factor leads to a good overview of ZT. For both p- and n-type individually separated into systems, ZTs and peak ZTs in dependence on the composition are displayed and discussed. This overview can help to find the ideal half-Heusler alloy for practical use.

1. Introduction

Thermoelectric (TE) modules and generators have the ability to directly convert waste heat into electric power and thus can be considered alternative, sustainable and “green” energy sources. There exist various classes of TE materials such as tellurides, selenides, clathrates, silicides, oxides, Zintl phases, antimonides, skutterudites, organic semiconductors, Heusler and half-Heusler alloys, etc., each of them qualifying in a defined temperature range. To judge the TE quality, the figure of merit, ZT = S2T/ρ(λe + λph), is used, where S is the Seebeck coefficient, ρ is the electrical resistivity, λ is the thermal conductivity, consisting of an electronic part, λe, and a phonon (lattice) part, λph, and T is the temperature. To arrive at a high ZT, the power factor, pf = S2/ρ, should be high, whereas λ should be low.
Half-Heusler (HH) alloys are promising TE materials intended for mid-to-high temperature power generation applications with already high ZT values. In addition, the starting materials are available, abundant, and, if hafnium and noble metals are avoided, cheap, a fact very important in respect of mass production.
Half-Heusler (HH) alloys are named after Friedrich Heusler (1866–1947), a German chemist and mining engineer. Half-Heusler alloys are members of the vast family of Heusler alloys with the general composition X2YZ, consisting of three interpenetrating face-centered cubic (fcc) sublattices (space group Fm 3 ¯ m). Reduction of symmetry (non-centrosymmetric space group F-43m) splits the X2 sublattice of multiplicity eight into two sublattices of multiplicity four, of which one is empty, resulting in the formula XYZ of the so-called HH phases. The big advantage in the optimization of thermoelectric properties of HH compounds is the opportunity to dope each of the four sublattices individually. Besides that, HH alloys tend from metallic to half-metallic alloys, exhibit interesting magnetic properties and are small band gap semiconductors. Generally, one can divide HH alloys into the following main groups: MNiSn, MCoSb (M = Ti, Zr, Hf), MFeSb (M = V, Nb, Ta), the 19-electron system and other HH alloys not covered by these four systems.
Reviews as papers, chapters in a book, or even a book have been published [1,2,3,4,5,6,7,8] focusing mainly on the crystal structure, magneto-optical properties, electron–phonon interactions, mobility, ways to enhance the power factor, new advances, applications and so on. Freer et al. [9] recently published tables of TE materials: the chapter about HH alloys was handled by S. Han, C. Fu and T. Zhu. They collected TE parameters from about 90 publications of p- and n-type HH alloys, however, from each publication for the compound with the highest ZT only (for room temperature and the temperature of the peak ZT). These peak ZTs were also displayed as a figure.
In the current review, measured data of the Seebeck coefficient, the electrical resistivity and the thermal conductivity of all compositions in each published paper were collected, as well as the calculated power factor and ZT (at 300 K and 800 K) and particularly the peak ZT. As more than 220 papers were evaluated, this adds up to about 300 p-type and 810 n-type compositions. These data were plotted and evaluated to gain a deeper insight, e.g., ZT vs. Seebeck coefficient, resistivity, thermal conductivity, composition and pf vs. thermal conductivity.

2. Experimental Production Methods

There exist various methods to produce half-Heusler alloys, the basics of the most popular ones will be described as follows.
The historically favored route, and still the mostly used one, is the melting-annealing one. With some variations, the essential steps are to arc-melt the defined amounts of high-purity elemental pieces under argon, flip the reguli and remelt them several times for good homogenization. Oxygen contamination must be avoided; therefore, a certain vacuum level must be kept in combination with high-purity argon (the use of oxygen-getter material, such as Zr or Ti, which is melted prior to the actual samples, can further improve the sample quality). In some cases, the samples are additionally melted in a high-frequency induction furnace several times. Between the melting steps, it is possible to frequently break them into pieces and turn them upside down and outside-inside. As a last step, they are wrapped into protective Mo foils, vacuum sealed in a quartz ampulla and annealed at high temperatures, usually between 900–1000 °C for at least 48 h to equilibrate the samples and to further increase homogeneity.
This method is also suitable for big quantities (ingots with a weight of >1 kg) when stoichiometric amounts of pure elements are synthesized together in a vacuum induction furnace, and the melt is cast into a mold.
A similar production method is levitation melting (e.g., [10,11,12]), which is becoming more and more popular in recent years. The main advantage of levitation melting is that (i) the sample has no contact with the environment as well as (ii) the eddy currents, which improve the homogeneity of the melt.
All these above-described methods are followed by breaking the solidified samples and pulverizing them manually or, in recent years more often practiced, ball-mill (BM) or high-energy ball-mill (HBM) and consolidating them either via hot-pressing (HP) or spark plasma sintering (SPS). For half-Heusler alloys containing materials with a high vapor pressure at elevated temperatures, such as antimony, it is necessary to compensate for antimony vaporization losses before HP.
Mechanical alloying (MA) is a fast method (e.g., [13,14,15,16]) to prepare homogeneous powder mixtures prior to consolidation in HP or spark plasma sintering devices.
In the so-called solid-state reaction process (e.g., [17]), the powder mixture of high-purity constituents is heated (almost 1000 °C) under a flowing argon atmosphere for up to a week, usually followed by SPS.
Self-propagation high-temperature synthesis (SHS) or combustion synthesis (CS) is a scalable, rapid preparation method. It is based on a propagating self-sustained exothermic reaction in consolidated elemental powders of the desired composition, and it yields a homogeneous product similar to the ones obtained by the well-known zone melting process. In addition, the SHS process is so fast that evaporation of low-melting-point elements is dramatically suppressed, leading to a precisely controlled composition (for details, see, e.g., [18,19,20]).
In some cases, SHS is combined with the selective laser melting (SLM) method. For SLM, a laser beam is used to melt a single-layer powder bed that rapidly solidifies as soon as the laser moves away. Layer upon layer, a three-dimensional object can be formed.
Microwave preparation (e.g., [21,22,23,24]) is another fast way to prepare half-Heusler alloys. Stoichiometric amounts of metal powders are cold-pressed into bar-shaped pellets, directly sealed into evacuated quartz tubes and placed into a crucible filled with some granular carbon, which acts as the microwave susceptor material. The sample is surrounded by alumina insulation foam to minimize heat loss. Finally, the whole set-up is placed into a commercial microwave reactor with a rotating plate. The reactions are allowed to run at 100% power (e.g., 700 W) for 1 min.
In some cases, the bulk HH alloy is processed by high-pressure torsion to further improve the TE performance by severe plastic deformation resulting in a drastic reduction towards ultra-low thermal conductivity, which occurs due to grain refinement and deformation-induced defects (for details, see references [25,26,27]).
Whenever the phonon part of the thermal conductivity was not published, it was calculated by the authors: λph = λ−λe with λ as the total thermal conductivity and λe as the electron part; λe = LT/ρ (Wiedemann–Franz law) with L as the Lorenz number (calculated as suggested by Kim et al. [28]) and ρ is the electrical resistivity. In all cases where the power factor and/or ZT were not published, it was calculated by the authors from pf = S2/ρ and ZT = S2T/(ρλ), respectively.

3. Results and Discussion

3.1. General Overview

In this chapter, all data from references [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218] were used for figures and discussions. The references [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218] are numbered by the publication year, within one year, in alphabetical order.
Looking at the highest ZTs published during the last 25 years, one can see in Figure 1 that, as already mentioned in the introduction, considerably more n-type half-Heusler alloys were investigated than p-type (this review comprises 294 p-type compounds and 816 n-type compounds). It is also clearly visible that the trend of high peak ZTs increases almost linearly from 2004 to 2018. There are exceptions for the n-type, such as (i) the work of Sakurada et al. [43], who published already in 2005 outstanding high values (ZT = 1.25 to 1.5) for Ti0.5Zr0.25Hf0.25NiSn doped with small amounts of antimony, or (ii) from Shen et al. [17,33] and Kawasaki et al. [37], who reached almost ZTs = 1 at 800 K in 2001 and 2004 for ZrNiSn0.99Sb0.01. It is also worth mentioning that Yu et al. [150] reached ZT = 1.6 at 1200 K for his p-type HH series (Nb1−xTax)0.8Ti0.2FeSb. Furthermore, it seems that after a boom during 2015–2020, the number of half-Heusler publications has dropped significantly.
Figure 2 displays the highest ZTs as a function of temperature. It is necessary to point out that in some cases ZTmax is not really the peak ZT of the respective sample but the ZT at the highest measured temperature. This is especially the case for publications earlier than 2005. Besides that, in most cases ZTmax is published for temperatures between 700 K and 1000 K; the highest temperatures for ZTmax for p- as well as n-type are around 1200 K.
Figure 3 and Figure 4 depict the dependence of the power factor on the thermal conductivity at 300 K and 800 K, respectively. Figure 3 shows that at 300 K, almost all power factor values for the n-type are within the range of 4 mW/mK2 for thermal conductivities up to 250 mW/cmK, whereas the p-type displays much higher power factors, reaching values higher than 10 mW/mK2 for thermal conductivities up to about 150 mW/cmK, e.g., for the Nb1−xTixFeSb-series of He et al. [131] or for Nb0.95Hf0.05FeSb and Nb0.95Zr0.05FeSb of Ren and colleagues [160].
The situation has changed comparing this figure now with the one for 800 K (Figure 4), and as for p- and n-type, all data are mainly evenly distributed within a power factor–thermal conductivity window of almost 0–5.5 mW/mK2 by 20–80 mW/cmK.
Lines for the resulting ZT values are drawn in both Figure 3 and Figure 4, revealing that at 300 K, no compound exceeds ZT = 0.5. Whilst many of the p-type compounds range between ZT = 0.1 and 0.25 or have values as low as almost 0, one can find the values for the n-type all over, the very low values due to high thermal conductivities. At 800 K, both p- and n-type exceed the ZT = 1.5 line. The majority of data can be found for thermal conductivities between 20 and 80 mW/cmK, representing ZTs from almost 0 to ZT = 1.5. This figure also illustrates that p-type HH materials generally exhibit lower thermal conductivities than the n-type.
Figure 5 and Figure 6 depict the relation between electrical resistivity and ZT at 300 K and 800 K, respectively. It shows that, especially in the 300 K temperature region, individual specimens have very high resistivities, resulting, as a consequence, in very low ZTs. Generally, the electrical resistivity decreases with increasing temperature. Figure 6 displays lower resistivity values. Still, here one can find ZTs in the range of 10−3 or even lower. The insert, a cutout, in each ZT-resistivity figure, gives a better overview, indicating that, especially for 800 K, most data can be found between 500 and 2500 µΩcm. In this region are also the highest ZTs.
Figure 7 and Figure 8 present the dependence of ZT on the Seebeck coefficient. Especially at 300 K, the area of Seebeck values is much wider for the n- than for the p-type. Generally, it seems that an absolute Seebeck value of 200 ± 20 µV/cm leads to high ZTs, a great finding for the production of new half-Heuslers.
Whereas for the high-temperature regime (Figure 9), the data ZT vs. λ are more or less evenly mixed at 300 K (insert in Figure 9), most p-type half-Heuslers have thermal conductivities between 20 and 70 mW/cmK. Of course, in both regions, ZTs are high for low thermal conductivities. Comparing the total and the lattice thermal conductivities in Figure 9 and Figure 10, the distribution of the data looks very similar, indicating that the electron part of the thermal conductivity is rather low, and, as a consequence, the phonon part is in the range of the total thermal conductivity.
In this chapter, it was only distinguished between p- and n-type half-Heusler alloys. In the following paragraphs for p- and n-type half-Heusler alloys, in addition, the various systems, i.e., the (Ti,Zr,Hf)NiSn-system, the (Ti,Zr,Hf)CoSb-system, the (V,Nb,Ta)FeSb-system, the 19-electron system and other p- and n-type HH materials (see also Table 1 and Table 2) will be discussed separately.

3.2. N-Type Half-Heusler Alloys

All compounds with the respective references can be found in Table 1. In some cases (indicated by *), measurements were only performed below room temperature.

3.2.1. (Ti,Zr,Hf)NiSn-System

Comparing the ZT values of TiNiSn, ZrNiSn and HfNiSn, as shown in Figure 11 and Figure 12, one can immediately see that ZrNiSn is the most investigated alloy of these three, whereas HfNiSn is not so popular. It is evident from Figure 11 and Figure 12 that the highest ZTs at 300 K as well as at 800 K in this system are for (Zr,Hf)NiSn and (Ti,Zr,Hf)NiSn and even more so when Sb-doped at the Sn site.
Gürth et al. [129] published ZT = 1 at 823 K for TiNiSn and for ZrNiSn, Misra et al. [106] ZT = 0.9 at 773 K for ZrNiSn and Liu et al. [119] ZT = 0.8 at 1000 K for HfNiSn. In comparison to these values, those for compounds with Ti and Zr, Ti and Hf, Zr and Hf or with Ti and Zr and Hf reach much higher values, such as for Zr0.7Hf0.3NiSn ZT = 1.2 at 873 K (Chauhan et al. [154]), for Ti0.5Zr0.25Hf0.25NiSn ZT = 1.3 at 693 K (Sakurada and Shutoh [43]; Shutoh and Sakurada [47]) or even ZT = 1.5 at 800 K (Rogl et al. [146]).
Doping in the (Ti,Zr,Hf)NiSn-system with X (X = Sc, Y, V, Nb, V + Nb, Ta, Mn, Co, Cu, Co + Cu, Pd, Pt, Pb) (see also Table 1) was not really successful as generally, all these alloys did not have higher ZTs; however, from Figure 11 as well as from Figure 12 it is obvious that with an additional substitution of Sn in many cases ZT is enhanced. Usually, Sn is substituted by a very small amount of Sb or Ge, but also, in rarer cases, In, Si, Pb, Bi or Te was used. The highest ZT values were reached for Ti0.5Zr0.25Hf0.25NiSn0.998Sb0.002 at 700 K (ZT = 1.5, Shutoh and Sakurada [47]) and Ti0.5Zr0.25Hf0.25NiSn0.99Sb0.01 at 825 K (ZT = 1.45, Rogl et al. [146]). Besides that, there are several compounds with ZT ∼ 1.3, such as (Hf0.6Zr0.4)0.99V0.01NiSn0.995Sb0.005 at 900 K (Chen et al. [140]) or Ti0.49Zr0.49V0.02NiSn0.98Sb0.02 at 823 K (Rogl et al. [146]) (see also Figure 12).
Various groups used additions (not shown in any figure but presented in Table 1), mainly nanoparticles, to reduce the thermal conductivity and to enhance ZT. Schwall and Balke [80] incorporated Nb into Zr0.5Hf0.5NiSn but presented data only below room temperature. Akram et al. [108] admixed La with ZrNiSn: the enhancement in ZT was within the error bar of ZT (from 0.52 to 0.54), which also applies to Visconti et al. [149], who used HfO2 for Ti0.5Zr0.5NiSn0.994Sb0.006 (ZT rises from 0.9 to 1). A better result was achieved by Chauhan et al. [153] by mixing ZnO with ZrNiSn (highest ZT = 1). Nanoparticles of ZrO2 were added to ZrNiSn (Huang et al. [35]) as well as to Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01 and Zr0.25Hf0.6Ti0.15NiSn0.995Sb0.005 (Chen et al. [49] and [127], respectively). For ZrNiSn, the “enhancement” of ZT was within the error bar; however, it was much better for the other two compounds with ZTmax = 0.74 and even ZTmax = 1.3 for the Ti-doped half-Heusler alloy. Additionally, a rather high ZTmax = 1.35 at 873 K was achieved by Kang et al. [197] for Zr0.4Hf0.6NiSn0.99Sb0.01 plus 2 weight percent tungsten nanoparticles. Makongo et al. [77,78] and Douglas et al. [83] added to Zr0.25Hf0.75NiSn and TiNiSn the corresponding full Heusler phase, i.e., Zr0.25Hf0.75Ni2Sn and TiNi2Sn, respectively; however, the outcome was rather disappointing with low ZTs < 1, in some cases even lower than ZT without the full-Heusler phase.
Rogl et al. [25,26] processed arc-melted, annealed, ball-milled and hot-pressed Ti0.5Zr0.5NiSn and Ti0.5Zr0.5NiSn0.98Sb0.02 samples by high-pressure torsion to improve their thermoelectric performance via a drastic reduction of the thermal conductivity due to grain refinement and a high concentration of defects inferred by severe plastic deformation. Whereas for Ti0.5Zr0.5NiSn, the thermally stable alloy showed an enhancement of ZTmax of 20%, for Ti0.5Zr0.5NiSn0.98Sb0.02 ZTmax was, within the error bar, about the same.
As a summary, one can say that the (Ti,Zr,Hf)NiSn-system comprises TE materials of high interest.

3.2.2. (Ti,Zr,Hf)CoSb-System, (V,Nb,Ta)FeSb-System, 19-Electron System and Other N-Type Half-Heusler Alloys

In Figure 13 and Figure 14, the ZTs are shown for the (Ti,Zr,Hf)CoSb-system, the (V,Nb,Ta)FeSb-system, the 19-electron system and of all those half-Heuslers, which do not fit into any of the so-far mentioned categories. For all these systems, only a few half-Heuslers with Sb/Sn substitutions were published; generally, ZTs are much lower in comparison with the (Ti,Zr,Hf)NiSn-system for 300 K as well as for 800 K.
At 800 K (Figure 14), almost all ZTs are lower than 0.7 and also ZTmax rarely exceeds 1. He et al. [141] reached ZT > 1 for Zr0.5Hf0.5Co0.9Ni0.1Sb at 1073 K, Liu et al. [158] acquired ZTs of almost 1 and around 1 for the series (Zr1−xHfx)0.88Nb0.12CoSb. Xia et al. [116,165] investigated Nb1−xCoSb and Nb0.8Co1−xNixSb with ZT = 0.8–0.9. Zhu et al. [187] obtained ZT = 0.85 at 800 K with a peak ZT = 1.04 at 972 K for ZrCo0.9Ni0.1Bi0.85Sb0.15.

3.3. P-Type Half-Heusler Alloys

All compounds with the respective references can be found in Table 2. In some cases, measurements were only performed below room temperature. Figure 15 and Figure 16 represent the ZTs for p-type half-Heusler alloys at 300 K and 800 K as well as the peak ZTs, respectively.
When looking at Section 3.3, it is obvious that many more n-type than p-type alloys were investigated. As depicted in Figure 15, at 300 K, all ZTs are below 0.3; at 800 K (Figure 16), most ZTs are lower than 1; however, several half-Heuslers in the (NbX’’)FeSb series with X’’ = Ti, Zr, Hf, Zr and Hf, as well as Ti + Ta, reach ZT = 1.4 at 800 K; the values for ZTmax in some cases reach or even exceed 1.4 and go up to 1.6.
It can be seen in Figure 15 and Figure 16 that only data of two p-type compounds are available for the (Ti,Zr,Hf)NiSn-system, namely (Zr,Sc)NiSn [122] and (Zr,Y)NiSn [114] with rather low ZTs.
In the CoSb-based system, almost all compounds are substituted at the Sb-site by Sn, except for TiCoSb, which was also substituted by Ge [68]. Rausch et al. published in two separate works [120,121] rather high ZTs of the (Ti,Zr,Hf)CoSb1−xSnx, series: the highest ZTmax = 1.4 for Ti0.12Zr0.44Hf0.44CoSb0.8Sn0.2 at 963 K [121].
In the FeSb-based system, Yu et al. [150] achieved outstanding high peak ZT values (ZT = 1.2–1.6) for the series Ti0.2(Nb1−xTax)0.8FeSb; Zhu et al. gained ZT~1.4 at 1200 K for Hf0.12Nb0.88FeSb [168] and for Ti0.16Ta0.84FeSb [188].
Whereas many p-type half-Heuslers have ZTs < 1, the ZrCoBi-based half-Heuslers showed excellent thermoelectric performance, even more so for substituting at the Bi-site with ZT = 1.42 for ZrCoBi0.65Sb0.15Sn0.20 at 973 K as reported by Zhu et al. [169].
Also, for p-type half-Heusler alloys, some research groups tried to enhance ZT via additions. Kimura et al. [71] added Co and Ir to ZrNiSn and converted this way the n-type to a p-type half-Heusler alloy; however, ZTs were rather low, i.e., below 0.3. Mallick et al. [159] investigated (Hf0.7Ti0.3CoSb0.8Sn0.2)1−x(Cu1.98Ni0.04Te0.97Se0.03)x: for x = 0.25 they could enhance ZT = 0.15 (without the chalcogenide) to ZT = 0.63 at 1023 K, which corresponds an enhancement of 320%. Hsu et al. [103] were successful in producing nanostructured HfO2 during the ball-milling and spark–plasma–sintering process of Zr0.5Hf0.5CoSb0.8Sn0.2 and reached as highest ZT = 0.75. Xie et al. [193] enhanced ZT of Ti(Co,Fe)Sb with the addition of 1 atom percent nano inclusions of InSb by 450%; however, ZT = 0.33 is still very low.
Not only for n-type half-Heusler alloys, as described in Section 3.2.1, but also for the p-type, NbFeSb and Ti0.15Nb0.85FeSb, the influence of severe plastic deformation via high-pressure torsion was investigated by Rogl and colleagues [25,26]. For NbFeSb, the peak ZT could be enhanced by about 50% but is, with ZT = 0.00043, still very low. For Ti0.15Nb0.85FeSb, ZT was enhanced from ZT = 0.68 to ZT = 0.74 for the thermally stable sample.
Generally, for n- as well as for p-type half-Heuslers, most ZTs at 300 K are below 0.3. Above 300 K, many peak ZTs are below 1 but can reach 1.6. Whereas the most successful system for the n-type is the (Ti,Zr,Hf)NiSn-system with Sb/Sn substitution, for the p-type, it is the (V,Nb,Ta)FeSb-system doped with Ti, Zr or Hf.

3.4. Miscellaneous

On 8-electron half-Heusler alloys: Several authors have screened 8-electron half-Heusler alloys in DFT throughput calculations, identifying a series of compounds of interest in thermoelectrics. Vikram et al., in one publication [219], presented the TE properties of 21 half-Heusler alloys with a sum of 8 valence electrons (8-electron half-Heuslers); 6 of them showed ZTs between 0.8 and 1, such as LiSnAl, NaAlSn or LiAlGe. In another paper [220], he calculated the thermoelectric properties of a large number of ternary compounds of the type: group IA or IB element—group IIA or IIB element—group V element. From these results, he highlighted even ZTs such as ZT = 1.37 for RbPBa and ZT = 1.56 for AgPMg. Hoat et al. [221] also made theoretical investigations of the 8-electron half-Heusler alloy RbYSn, but they published only the power factors for the p- and n-types. So far, however, no experimental confirmation exists for the ZTs estimated from these DFT calculations.
On “double half-Heusler alloys”, etc.: Some authors have coined the term “double half-Heusler alloys”, “triple half-Heusler alloys”, etc. These terms are not without problems. Firstly, double a half is full, but the authors certainly did not want to discuss full-Heusler alloys. Secondly, a formula such as Ti2FeCoSnSb from a structural chemical point of view indicates a fully ordered compound, where Fe,Co and Sn,Sb atoms fully occupy separate two-fold sites in a crystallographic subgroup of the space group of the half-Heusler phase. As such a subgroup cannot be found among the HH face-centered cubic space group type, the structure, therefore, needs to be transferred into a suitable body-centered tetragonal space group (cubic setting a,a,a needs to be transferred to a tetragonal cell: a/ 2 , a/ 2 , a ) in order to find sites of two-fold multiplicity. Only if authors can prove the full atom order will the formula Ti2FeCoSnSb be proper. Similar arguments hold for “triple half-Heusler alloys“, etc.

4. Conclusions

The review presented here attempts to cover all measured thermoelectric properties of p- and n-type half-Heusler alloys of the last 25 years. Of course, even with the best literature search, it may be possible to miss some publications. It also became obvious that there exist about twice as many papers of n-type than of p-type half-Heusler alloys.
As the figure of merit, ZT is dependent on three key figures, the electrical resistivity, the Seebeck coefficient and the thermal conductivity, the evaluations in this review, focusing on these key figures, may help to understand the interrelations between them as well as their influence on ZT.
It was found out that the n-type (Ti,Zr,Hf)NiSn-sytem generally has the highest ZTs and that (Ti,Zr)NiSn, (Ti,Hf)NiSn, (Zr,Hf)NiSn and especially (Ti,Zr,Hf)NiSn have higher ZTs than TiNiSn, ZrNiSn and HfNiSn. Furthermore, this investigation showed that doping, in most cases, does not enhance ZT as much as substituting at the antimony site, preferably tin by antimony. Various additions of nanoparticles could, in some cases, push up ZT slightly.
For the p-type, the (V,Nb,Ta)FeSb-system in respect of high ZT is the best and even better with compounds doped with Ti or Hf, but also quite remarkable ZTs were found for the (Ti,Zr,Hf)CoSb-system substituted by tin, and for ZrCoBi substituted at the bismuth site by antimony and tin.
All these evaluated ZTs are, of course, primarily dependent on the preparation method warranting a proper microstructure with small (preferably nano) grain size and a high density.
The present compilation may provide the data for the individual selection of p- and n-type leg materials suitable for an efficient thermoelectric generator within a given temperature gradient.

Author Contributions

Both authors (G.R. and P.F.R.) equally contributed to the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data sheet, which served as the basis for figures and tables, can be obtained from the authors upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Poon, S.J. Electronic and thermoelectric properties of half-Heusler alloys. In Semiconductors and Semimetals; Elsevier: Amsterdam, The Netherlands, 2001; Volume 70, pp. 37–72. [Google Scholar]
  2. Graf, T.; Felser, C.; Parker, S.S.P. Simple rules for the understanding of Heusler compounds. Prog. Solid State Chem. 2011, 39, 1–50. [Google Scholar] [CrossRef]
  3. Lee, M.-S.; Poudeu, F.P.; Mahanti, S.D. Electronic structure and thermoelectric properties of Sb-based semiconducting half-Heusler compounds. Phys. Rev. B 2011, 83, 085204. [Google Scholar] [CrossRef] [Green Version]
  4. Xie, W.; Weidenkaff, A.; Tang, X.; Zhang, Q.; Poon, J.; Tritt, T.M. Recent advances in nanostructured thermoelectric half-Heusler compounds. Nanomaterials 2012, 2, 379–412. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  5. Felser, C.; Hirohata, A. Heusler Alloys: Properties, Growth, Applications. In Springer Series in Materials Science; Springer: Cham, Switzerland, 2016; Volume 222, p. 2015945328. [Google Scholar]
  6. Zhu, T.; Liu, Y.; Fu, C.; Heremans, J.P.; Snyder, J.G.; Zhao, X. Compromise and Synergy in High-Efficiency Thermoelectric Materials. Adv. Mater. 2017, 20, 1605884. [Google Scholar] [CrossRef] [PubMed]
  7. Zhou, J.; Zhu, H.; Liu, T.-H.; Song, Q.; He, R.; Mao, J.; Liu, Z.; Ren, W.; Liao, B.; Singh, D.J.; et al. Large thermoelectric power factor from crystal symmetry-protected non-bonding orbital in half-Heuslers. Nat. Commun. 2018, 9, 1721–1730. [Google Scholar] [CrossRef] [Green Version]
  8. Quinn, R.J.; Bos, J.-W.G. Advances in half-Heusler alloys for thermoelectric power generation. Mater. Adv. 2021, 19, 6246–6266. [Google Scholar] [CrossRef]
  9. Freer, R.; Ekren, D.; Ghosh, T.; Biswas, K.; Qiu, P.; Wan, S.; Chen, L.; Han, S.; Fu, C.; Zhu, T.; et al. Key properties of inorganic thermoelectric materials—Tables. J. Phys. Energy 2022, 4, 022002. [Google Scholar] [CrossRef]
  10. Hu, T.; Yang, D.; Su, X.; Yan, Y.; You, Y.; Liu, W.; Uher, C.; Tang, X. Interpreting the combustion process for high-performance ZrNiSn thermoelectric materials. ACS Appl. Mater. Interfaces 2018, 10, 864–872. [Google Scholar] [CrossRef]
  11. Yu, C.; Zhu, T.-J.; Shi, R.-Z.; Zhang, Y.; Zhao, X.-B.; He, J. High-performance half-Heusler thermoelectric materials Hf1−x ZrxNiSn1−ySby prepared by levitation melting and spark plasma sintering. Acta Mater. 2009, 57, 2757–2764. [Google Scholar] [CrossRef]
  12. Yu, C.; Zhu, T.-J.; Xia, K.; Shen, J.J.; Yang, S.H.; Zhao, X. Reduced grain size and improved thermoelectric properties of melt spun (Hf, Zr) NiSn half-Heusler alloys. J. Electron. Mater. 2010, 39, 2008–2010. [Google Scholar] [CrossRef]
  13. Karati, A.; Ghosh, S.; Mallik, R.C.; Shabadi, R.; Murty, B.S.; Varadaraju, U.V. Effect of processing routes on the microstructure and thermoelectric properties of half-Heusler TiFe0.5Ni0.5Sb1−xSnx (x = 0, 0.05, 0.1, 0.2) alloys. J. Mater. Eng. Perform. 2022, 31, 305–317. [Google Scholar] [CrossRef]
  14. Bahrami, A.; Ying, P.; Wolff, U.; Perez-Rodriguez, N.; Schierning, G.; Nielsch, K.; He, R. Reduced lattice thermal conductivity for half-Heusler ZrNiSn through cryogenic mechanical alloying. ACS Appl. Mater. Interfaces 2021, 13, 38561–38568. [Google Scholar] [CrossRef] [PubMed]
  15. Hasan, R.; Ur, S.-C. Thermoelectric and transport Properties of V1−xTixFeSb half-Heusler system synthesized by controlled mechanical alloying process. Electron. Mater. Lett. 2018, 14, 725–732. [Google Scholar] [CrossRef]
  16. He, R.; Zhu, T.; Ying, P.; Chen, J.; Giebeler, L.; Kühn, U.; Grossman, J.C.; Wang, Y.; Nielsch, K. High-pressure-sintering-induced microstructural engineering for an ultimate phonon scattering of thermoelectric half-Heusler compounds. Small 2021, 17, 2102045. [Google Scholar] [CrossRef] [PubMed]
  17. Shen, Q.; Zhang, L.; Chen, L.; Goto, T.; Hirai, T. Thermoelectric properties of ZrNiSn-based half-Heusler compounds. J. Mater. Sci. Lett. 2001, 20, 2197–2199. [Google Scholar] [CrossRef]
  18. Xing, Y.; Liu, R.; Liao, J.; Zhang, Q.; Xia, X.; Wang, C.; Huang, H.; Chu, J.; Gu, M.; Zhu, T.; et al. High-efficiency half-Heusler thermoelectric modules enabled by self-propagating synthesis and topologic structure optimization. Energy Environ. Sci. 2019, 12, 3390–3399. [Google Scholar] [CrossRef]
  19. Xing, Y.F.; Liu, R.H.; Sun, Y.Y.; Chen, F.; Zhao, K.P.; Zhu, T.J.; Bai, S.Q.; Chen, L.D. Self-propagation high-temperature synthesis of half-Heusler thermoelectric materials: Reaction mechanism and applicability. J. Mater. Chem. A 2018, 6, 19470–19478. [Google Scholar] [CrossRef]
  20. Hu, T.; Cao, W.; Yang, D.; Yan, Y.; Cao, Y.; Zhang, T.; Su, X.; Liu, W.; Poudeu-Poudeu, P.; Tang, X. Ultra-fast fabrication of bulk ZrNiSn thermoelectric material through self-propagating high-temperature synthesis combined with in-situ quick pressing. Scripta Mater. 2019, 165, 140–144. [Google Scholar] [CrossRef]
  21. Birkel, C.S.; Zeier, W.G.; Douglas, J.E.; Lettiere, B.R.; Mills, C.E.; Seward, G.; Birkel, A.; Snedaker, M.L.; Zhang, Y.; Snyder, G.J.; et al. Rapid Microwave Preparation of Thermoelectric TiNiSn and TiCoSb half-Heusler Compounds. Chem. Mater. 2012, 24, 2558–2565. [Google Scholar] [CrossRef] [Green Version]
  22. Birkel, C.S.; Douglas, J.E.; Lettiere, B.R.; Seward, G.; Verma, N.; Zhang, Y.; Pollock, T.M.; Seshadri, R.; Stucky, G.D. Improving the thermoelectric properties of half-Heusler TiNiSn through inclusion of a second full Heusler phase: Microwave preparation and spark plasma sintering of TiNi1+xSn. Phys. Chem. Chem. Phys. 2013, 15, 6990–6997. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  23. Lei, Y.; Li, Y.; Xu, L.; Yang, J.; Wan, R.; Long, H. Microwave synthesis and sintering of TiNiSn thermoelectric bulk. J. Alloys Compd. 2016, 660, 166–170. [Google Scholar] [CrossRef]
  24. Lei, Y.; Cheng, C.; Li, Y.; Wan, R.; Mang, M. Microwave synthesis and enhancement of thermoelectric figure of merit in half-Heusler TiNiSbxSn1−x. Ceram. Int. 2017, 43, 9343–9347. [Google Scholar] [CrossRef]
  25. Rogl, G.; Zehetbauer, M.J.; Rogl, P.F. The effect of severe plastic deformation on thermoelectric performance of skutterudites, half-Heuslers and Bi-tellurides. Mater. Trans. 2019, 60, 2071–2085. [Google Scholar] [CrossRef] [Green Version]
  26. Rogl, G.; Ghosh, S.; Wang, L.; Bursik, J.; Grytsiv, A.; Mallik, R.C.; Chen, X.-Q.; Zehetbauer, M.; Rogl, P. Half-Heusler alloys: Enhancement of ZT after severe plastic deformation (ultra-low thermal conductivity). Acta Mater. 2020, 183, 285–300. [Google Scholar] [CrossRef]
  27. Rogl, G.; Rogl, P.F. How severe plastic deformation changes the mechanical properties of thermoelectric skutterudites and half-Heusler alloys. Front. Mater. 2020, 7, 600261. [Google Scholar] [CrossRef]
  28. Kim, H.-S.; Gibbs, Z.M.; Tang, Y.; Wang, H.; Snyder, G.J. Characterization of Lorenz number with Seebeck coefficient measurement. APL Mater. 2015, 3, 041506. [Google Scholar] [CrossRef] [Green Version]
  29. Browning, V.M.; Poon, S.J.; Tritt, T.M.; Pope, A.L.; Bhattacharya, S. Thermoelectric properties of the half-Heusler compound (Zr, Hf)(Ni, Pd) Sn. Proc. MRS Fall 1998, 545, 403. [Google Scholar] [CrossRef]
  30. Cook, B.A.; Meisner, G.P.; Yang, J.; Uher, C. High temperature thermoelectric properties of MNiSn (M = Zr, Hf). In Proceedings of the Eighteenth International Conference on Thermoelectrics, Baltimore, MD, USA, 29 August 1999; pp. 64–67. [Google Scholar]
  31. Hohl, H.; Ramirez, A.P.; Goldmann, C.; Ernst, G.; Wölfing, B.; Bucher, E. Efficient dopants for ZrNiSn-based thermoelectric materials. J. Phys. Condens. Matter 1999, 11, 1697–1709. [Google Scholar] [CrossRef]
  32. Uher, C.; Yang, J.; Hu, S.; Morelli, D.T.; Meisner, G.P. Transport properties of pure and doped MNiSn, M = Zr, Hf. Phys. Rev. B 1999, 59, 8615–8621. [Google Scholar] [CrossRef]
  33. Shen, Q.; Chen, L.; Goto, T.; Hirai, T.; Yang, J.; Meisner, G.P.; Uher, C. Effects of partial substitution of Ni by Pd on the thermoelectric properties of ZrNiSn based half-Heusler compounds. Appl. Phys. Lett. 2001, 79, 4165–4172. [Google Scholar] [CrossRef] [Green Version]
  34. Katayama, T.; Kim, S.W.; Kimura, Y.; Mishima, Y. The effects of quaternary additions on thermoelectric properties of TiNiSn-based half-Heusler alloys. J. Electron. Mater. 2003, 32, 1160–1165. [Google Scholar] [CrossRef]
  35. Huang, X.Y.; Xu, Z.; Chen, L.D. The thermoelectric performance of ZrNiSn/ZrO2 composites. Solid State Commun. 2004, 130, 181–185. [Google Scholar] [CrossRef]
  36. Katsuyama, S.; Matsushima, H.; Ito, M. Effect of substitution for Ni by Co and/or thermoelectric properties of half-Heusler ZrNiSn. J. Alloys Compd. 2004, 385, 232–237. [Google Scholar] [CrossRef]
  37. Kawaharada, Y.; Uneda, H.; Muta, H.; Kurosaki, K.; Yamanaka, S. High temperature thermoelectric properties of NiZrSn half-Heusler compounds. J. Alloys Compd. 2004, 364, 59–63. [Google Scholar] [CrossRef]
  38. Kawaharada, Y.; Kurosaki, K.; Muta, H.; Uno, M.; Yamanaka, S. High temperature thermoelectric properties of TiCoSb half-Heusler compounds. J. Alloys Compd. 2004, 384, 308–311. [Google Scholar] [CrossRef]
  39. Kim, S.W.; Kimura, Y.; Mishima, Y. Enhancement of high temperature thermoelectric properties of intermetallic compounds based on a skutterudite IrSb3 and a half-Heusler TiNiSb. Sci. Technol. Adv. Mater. 2004, 5, 485–489. [Google Scholar] [CrossRef]
  40. Kurosaki, K.; Muta, H.; Yamanaka, S. Thermoelectric properties of titanium-based half-Heusler compounds. J. Alloys Compd. 2004, 384, 51–56. [Google Scholar] [CrossRef]
  41. Kurosaki, K.; Maekawa, T.; Muta, H.; Yamanaka, S. Effect of spark plasma sintering temperature on thermoelectric properties of (Ti, Zr, Hf)NiSn half-Heusler compounds. J. Alloys Compd. 2005, 397, 296–299. [Google Scholar] [CrossRef]
  42. Muta, H.; Yamaguchi, T.; Kurosaki, K.; Yamanaka, S. Thermoelectric properties of ZrNiSn based half-Heusler compounds. In Proceedings of the Twentyfourth International Conference on Thermoelectrics, Clemson, SC, USA, 19 August 2005; pp. 339–342. [Google Scholar]
  43. Sakurada, S.; Shutoh, N. Effect of Ti substitution on the thermoelectric properties of (Zr,Hf)NiSn half-Heusler compounds. Appl. Phys. Lett. 2005, 86, 082105. [Google Scholar] [CrossRef]
  44. Sekimoto, T.; Kurosaki, K.; Muta, H.; Yamanaka, S. Annealing effect on thermoelectric properties of TiCoSb half-Heusler compound. J. Alloys Compd. 2005, 394, 122–125. [Google Scholar] [CrossRef]
  45. Sekimoto, T.; Kurosaki, K.; Muta, H.; Yamanaka, S. Thermoelectric properties of (Ti,Zr,Hf)CoSb type half-Heusler compounds. Mater. Trans. 2005, 46, 1481–1484. [Google Scholar] [CrossRef] [Green Version]
  46. Sekimoto, T.; Kurosaki, K.; Muta, H.; Yamanaka, S. Thermoelectric and thermophysical properties of TiCoSb, ZrCoSb, HfCoSb prepared by SPS. In Proceedings of the Twentyfourth International Conference on Thermoelectrics, Clemson, SC, USA, 19 August 2005; pp. 335–339. [Google Scholar]
  47. Sakurada, S.; Shutoh, N. Thermoelectric properties of the Tix(Zr0.5Hf0.5)1−xNiSn half-Heusler compounds. J. Alloys Compd. 2005, 389, 204–208. [Google Scholar]
  48. Zhou, M.; Feng, C.; Chen, L.; Huang, X. Effects of partial substitution of Co by Ni on the high-temperature thermoelectric properties of TiCoSb-based half-Heusler compounds. J. Alloys Compd. 2005, 391, 194–197. [Google Scholar] [CrossRef]
  49. Chen, L.D.; Huang, X.Y.; Zhou, M.; Shi, X.; Zhang, W.B. The high temperature thermoelectric performances of Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01 alloy with nanophase inclusions. J. Appl. Phys. 2006, 99, 064305. [Google Scholar] [CrossRef]
  50. Culp, S.R.; Poon, S.J.; Hickman, N.; Tritt, T.M.; Blumm, J. Effect of substitutions on the thermoelectric figure of merit of half-Heusler phases at 800 °C. Appl. Phys. Lett. 2006, 88, 042106. [Google Scholar] [CrossRef]
  51. Kimura, Y.; Zama, A.; Mishima, Y. Thermoelectric properties of p-type half-Heusler compounds HfPtSn and ZrPtSn. In Proceedings of the Twentyfifth International Conference on Thermoelectrics, Vienna, Austria, 6 August 2006; pp. 115–119. [Google Scholar]
  52. Ono, Y.; Inayama, S.; Adachi, H.; Kajitani, T. Thermoelectric properties of doped half-Heuslers NbCoSn1−xSbx and Nb0.99Ti0.01CoSn1−xSbx. JPN J. Appl. Phys. 2006, 45, 8740–8743. [Google Scholar] [CrossRef]
  53. Sekimoto, T.; Kurosaki, K.; Muta, H.; Yamanaka, S. Thermoelectric and thermophysical properties of ErPdX (X = Sb and Bi) half-Heusler compounds. J. Appl. Phys. 2006, 99, 103701. [Google Scholar] [CrossRef]
  54. Sekimoto, T.; Kurosaki, K.; Muta, H.; Yamanaka, S. Thermoelectric properties of (Ti,Zr)CoSnxSb1−x half-Heusler compounds. In Proceedings of the Twentyfifth International Conference on Thermoelectrics, Vienna, Austria, 6 August 2006; pp. 128–131. [Google Scholar]
  55. Sekimoto, T.; Kurosaki, K.; Muta, H.; Yamanaka, S. Thermoelectric properties of Sn-doped TiCoSb half-Heusler compounds. J. Alloys Compd. 2006, 407, 326–329. [Google Scholar] [CrossRef]
  56. Sekimoto, T.; Kurosaki, K.; Muta, H.; Yamanaka, S. Thermoelectric and thermophysical properties of TiCoSb-ZrCoSb-HfCoSb pseudo ternary system prepared by spark plasma sintering. Mater. Trans. 2006, 47, 1445–1448. [Google Scholar] [CrossRef] [Green Version]
  57. Katsuyama, S.; Matsuo, R.; Ito, M. Thermoelectric properties of half-Heusler alloys Zr1−xYxNiSn1−ySby. J. Alloys Compd. 2007, 428, 262–267. [Google Scholar] [CrossRef]
  58. Kim, S.-W.; Kimura, Y.; Mishima, Y. High temperature thermoelectric properties of TiNiSn-based half-Heusler compounds. Intermetallics 2007, 15, 349–356. [Google Scholar] [CrossRef]
  59. Sekimoto, T.; Kurosaki, K.; Muta, H.; Yamanaka, S. Thermoelectric properties of half-Heusler Type LaPdBi and GdPdBi. Mater. Trans. 2007, 48, 2079–2082. [Google Scholar] [CrossRef] [Green Version]
  60. Sekimoto, T.; Kurosaki, K.; Muta, H.; Yamanaka, S. High-thermoelectric figure of merit realized in p-type half-Heusler compounds: ZrCoSnxSb1−x. JPN J. Appl. Phys. 2007, 46, L673–L675. [Google Scholar] [CrossRef]
  61. Wu, T.; Jiang, W.; Li, X.; Zhou, Y.; Chen, L. Thermoelectric properties of p-type Fe-doped TiCoSb half-Heusler compounds. J. Appl. Phys. 2007, 102, 103705. [Google Scholar] [CrossRef]
  62. Culp, R.S.; Simson, J.W.; Poon, S.J.; Ponnambalan, V.; Edwards, J.; Tritt, T.M. (Zr,Hf)Co(Sb,Sn) half-Heusler phases as high-temperature (>700 °C) p-type thermoelectric materials. Appl. Phys. Lett. 2008, 93, 022105. [Google Scholar] [CrossRef]
  63. Kimura, Y.; Tamura, Y.; Kita, T. Thermoelectric properties of directionally solidified half-Heusler compound NbCoSn alloys. Appl. Phys. Lett. 2008, 92, 012105. [Google Scholar] [CrossRef]
  64. Xie, W.; Jin, Q.; Tang, X. The preparation and thermoelectric properties of Ti0.5Zr0.25Hf0.25Co1−xNixSb half-Heusler compounds. J. Appl. Phys. 2008, 103, 043711. [Google Scholar] [CrossRef]
  65. Kimura, Y.; Ueno, H.; Mishima, Y. Thermoelectric properties of directionally solidified half-Heusler (Ma0.5Mb0.5)NiSn (Ma, Mb = Hf, Zr, Ti) alloys. J. Electron. Mater. 2009, 38, 934–939. [Google Scholar] [CrossRef]
  66. Muta, H.; Kanemitsu, T.; Kurosaki, K.; Yamanaka, S. High-temperature thermoelectric properties of Nb-doped MNiSn (M = Ti, Zr) half-Heusler compound. J. Alloys Compd. 2009, 469, 50–55. [Google Scholar] [CrossRef]
  67. Qiu, P.; Huang, X.; Chen, X.; Chen, L. Enhanced thermoelectric performance by the combination of alloying and doping in TiCoSb-based half-Heusler compounds. J. Appl. Phys. 2009, 106, 103703. [Google Scholar] [CrossRef]
  68. Wu, T.; Jiang, W.; Li, X.; Bai, S.; Liufu, S.; Chen, L. Effects of Ge doping on the thermoelectric properties of TiCoSb-based p-type half-Heusler compounds. J. Alloys Compd. 2009, 467, 590–594. [Google Scholar] [CrossRef]
  69. Kenjo, T.; Kimura, Y.; Mishima, Y. Phase stability and thermoelectric properties of half-Heusler compounds (Ti,M)NiSn (M = Zr, Hf). Mater. Res. Soc. Symp. Proc. 2010, 1218, 43–48. [Google Scholar] [CrossRef]
  70. Kim, I.-H.; Kwon, J.-C.; Lee, Y.-G.; Yoon, M.-S.; Ryu, S.-L.; Kim, W.-G.; Ur, S.-C. Synthesis and thermoelectric properties of half-Heusler Zr-Ni-Sn alloy processed by mechanically alloying and hot-pressing. In Materials Science Forum; Trans Tech Publications Ltd.: Stafa-Zurich, Switzerland, 2010; Volume 658, pp. 33–36. [Google Scholar]
  71. Kimura, Y.; Tanoguchi, T.; Kita, T. Vacancy site occupation by Co and Ir in half-Heusler ZrNiSn and conversion of the thermoelectric properties from n-type to p-type. Acta Mater. 2010, 58, 4354–4361. [Google Scholar] [CrossRef]
  72. Ouardi, S.; Fecher, G.H.; Balke, B.; Kozina, X.; Stryganyuk, G.; Felser, C. Electronic transport properties of electron- and hole-doped semiconducting C1bHeusler compounds: NiTi1-xMxSn (M = Sc, V). Phys. Rev. B 2010, 82, 085108. [Google Scholar] [CrossRef]
  73. Zhu, T.J.; Xiao, K.; Yu, C.; Shen, J.J.; Yang, S.H.; Zhou, A.J.; Zhao, X.B.; He, J. Effect of yttrium doping on the thermoelectric properties of Hf0.6Zr0.4NiSn0.98Sb0.02 half-Heusler alloys. J. Appl. Phys. 2010, 108, 044903. [Google Scholar] [CrossRef]
  74. Zou, M.; Li, J.F.; Guo, P.; Kita, T. Synthesis and thermoelectric properties of fine-grained VFeSb system half-Heusler compound polycrystals with high phase purity. J. Phys. D Appl. Phys. 2010, 43, 415403. [Google Scholar] [CrossRef]
  75. Gelbstein, Y.; Tal, N.; Yarmek, A.; Rosenberg, Y.; Dariel, M.P.; Ouardi, S.; Balke, B.; Felser, C.; Köhne, M. Thermoelectric properties of spark plasma sintered composites based on TiNiSn half-Heusler alloys. J. Mater. Res. 2011, 26, 1919–1924. [Google Scholar] [CrossRef]
  76. Joshi, G.; Yan, X.; Wang, H.; Liu, W.; Chen, G.; Ren, Z. Enhancement in thermoelectric figure-of-merit of an n-type half-Heusler compound by the nanocomposite approach. Adv. Energy Mater. 2011, 1, 643–647. [Google Scholar] [CrossRef]
  77. Makongo, J.P.A.; Misra, D.K.; Zhou, X.Y.; Pant, A.; Shabetai, M.R.; Su, X.L.; Uher, C.; Stokes, K.L.; Poudeu, P.F.P. Simultaneous large enhancements in thermopower and electrical conductivity of bulk nanostructured half-Heusler alloys. J. Am. Chem. Soc. 2011, 133, 18843–18852. [Google Scholar] [CrossRef]
  78. Makongo, J.P.A.; Misra, D.K.; Salvador, J.R.; Takas, N.J.; Wang, G.; Shabetai, M.R.; Pant, A.; Poudel, P.; Uher, C.; Stokes, K.L.; et al. Thermal and electronic charge transport in bulk nanostructured Zr0.25Hf0.75NiSn composites with full Heusler inclusions. J. Solid State Chem. 2011, 184, 2948–2960. [Google Scholar] [CrossRef]
  79. Poon, S.J.; Wu, D.; Zhu, S.; Xie, W.; Tritt, T.M.; Venkatasubramanian, P.T.; Venkatasubramanian, R. Half-Heusler phases and nanocomposites as emerging high-ZT thermoelectric materials. J. Mater. Res. 2011, 26, 2795–2802. [Google Scholar] [CrossRef]
  80. Schwall, M.; Balke, B. Niobium substitution in Zr0.5Hf0.5NiSn based Heusler compounds for high power factors. Appl. Phys. Lett. 2011, 98, 042106. [Google Scholar] [CrossRef]
  81. Simonson, J.W.; Wu, D.; Xie, W.J.; Tritt, T.M.; Poon, S.J. Introduction of resonant states and enhancement of thermoelectric properties in half-Heusler alloys. Phys. Rev. B 2011, 83, 235211. [Google Scholar] [CrossRef] [Green Version]
  82. Yan, X.; Joshi, G.; Liu, W.; Lan, Y.; Wang, H.; Lee, S.; Simonson, J.W.; Poon, S.J.; Tritt, T.M. Enhanced thermoelectric figure of merit of p-type half-Heuslers. Nano Lett. 2011, 11, 556–560. [Google Scholar] [CrossRef] [PubMed]
  83. Douglas, J.E.; Birkel, C.S.; Miao, M.-S.; Torbet, C.J.; Stucky, G.D.; Pollock, T.M.; Seshadri, R. Enhanced thermoelectric properties of bulk TiNiSn via formation of a TiNi2Sn second phase. Appl. Phys. Lett. 2012, 101, 183902. [Google Scholar] [CrossRef]
  84. Populoh, S.; Aguirre, M.H.; Brunko, O.C.; Galazka, K.; Lu, Y.; Weidenkaff, A. High figure of merit in (Ti,Zr,Hf)NiSn half-Heusler alloys. Scripta Mater. 2012, 66, 1073–1107. [Google Scholar] [CrossRef]
  85. Xie, H.-H.; Mi, J.-L.; Hu, L.-P.; Lock, N.; Chirstensen, M.; Fu, C.-G.; Brummerstedt Iversen, B.; Zhao, X.-B.; Zhu, T.-J. Interrelation between atomic switching disorder and thermoelectric properties of ZrNiSn half-Heusler compounds. CrystEngComm 2014, 14, 4467–4471. [Google Scholar] [CrossRef]
  86. Yan, X.; Liu, W.; Wang, H.; Chen, S.; Shiomi, J.; Esfarjani, K.; Wang, H.; Wang, D.; Chen, G.; Ren, Z. Stronger phonon scattering by larger differences in atomic mass and size in p-type half-Heuslers Hf1−xTixCoSb0.8Sn0.2. Energy Environ. Sci. 2012, 5, 7543–7548. [Google Scholar] [CrossRef]
  87. Appel, O.; Schwall, M.; Mogilyansky, D.; Köhne, M.; Balke, B.; Gelbstein, Y. Effects of microstructural evolution on the thermoelectric properties of spark-plasma-sintered Ti0.3Zr0.35Hf0.35NiSn half-Heusler compound. J. Electron. Mater. 2013, 42, 1340–1345. [Google Scholar] [CrossRef]
  88. Birkel, C.S.; Douglas, J.E.; Lettiere, B.R.; Seward, G.; Zhang, Y.; Pollock, T.M.; Seshadri, R.; Stucky, G.D. Influence of Ni nanoparticle addition and spark plasma sintering on the TiNiSn-Ni system: Structure, microstructure, and thermoelectric properties. Solid States Sci. 2013, 26, 16–22. [Google Scholar] [CrossRef] [Green Version]
  89. Chen, S.; Lukas, K.C.; Liu, W.; Opeil, C.P.; Chen, G.; Ren, Z. Effect of Hf concentration on thermoelectric properties of nanostructured n-type half-Heusler materials HfxZr1−xNiSn0.99Sb0.01. Adv. Energy Mater. 2013, 3, 1210–1214. [Google Scholar] [CrossRef]
  90. Downie, R.A.; MacLaren, D.A.; Smith, R.I.; Bos, J.W.G. Enhanced thermoelectric performance in TiNiSn-based half-Heuslers. Chem. Commun. 2013, 49, 4184–4186. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  91. Fu, C.; Xie, H.; Liu, Y.; Zhu, T.; Xie, J.; Zhao, X.B. Thermoelectric properties of FeVSb half-Heusler compounds by levitation melting and spark plasma sintering. Intermetallics 2013, 32, 39–43. [Google Scholar] [CrossRef]
  92. Fu, C.; Liu, Y.; Xie, H.; Liu, X.; Zhao, X.; Snyder, J.G.; Xie, J.; Zhu, T. Electron and phonon transport in Co-doped FeV0.6Nb0.4Sb half-Heusler thermoelectric materials. J. Appl. Phys. 2013, 114, 134905. [Google Scholar] [CrossRef]
  93. Schwall, M.; Balke, B. Phase separation as a key to a thermoelectric high efficiency. Phys. Chem. Chem. Phys. 2013, 15, 1868–1872. [Google Scholar] [CrossRef]
  94. Xie, W.J.; Yan, Y.G.; Zhu, S.; Zhou, M.; Populoh, S.; Galazka, K.; Poon, S.J.; Weidenkaff, A.; He, J.; Tang, X.F.; et al. Significant ZT enhancement in p-type Ti(Co,Fe)Sb-InSb nanocomposites via a synergistic high-mobility electron injection, energy-filtering and boundary-scattering approach. Acta Mater. 2013, 61, 2087–2094. [Google Scholar] [CrossRef] [Green Version]
  95. Xie, W.J.; Wang, H.; Pei, Y.; Fu, C.; Liu, X.; Snyder, J.G.; Zhao, X.; Zhu, T. Beneficial contribution of alloy disorder to electron and phonon transport in half-Heusler thermoelectric materials. Adv. Funct. Mater. 2013, 23, 5123–5130. [Google Scholar] [CrossRef]
  96. Yan, X.; Liu, W.; Chen, S.; Wan, H.; Zhang, Q.; Chen, G.; Ren, Z. Thermoelectric property study of nanostructured p-type half-Heuslers (Hf,Zr,Ti)CoSb0.8Sn0.2. Adv. Energy Mater. 2013, 3, 1195–1200. [Google Scholar] [CrossRef]
  97. Zou, M.; Li, J.F.; Kita, T. Thermoelectric properties of fine-grained FeVSb half-Heusler alloys tuned to p-type by substituting vanadium with titanium. J. Solid State Chem. 2013, 198, 125–130. [Google Scholar] [CrossRef]
  98. Bartholome, K.; Balke, B.; Zuckermann, D.; Köhne, M.; Müller, M.; Tarantik, K.; König, J. Thermoelectric modules based on half-Heusler materials produced in large quantities. J. Electron. Mater. 2014, 43, 1775–1781. [Google Scholar] [CrossRef]
  99. Downie, R.A.; Mac Laren, D.A.; Bos, J.-W.G. Thermoelectric performance of multiphase XNiSn (X = Ti, Zr, Hf) half-Heusler alloys. J. Mater. Chem. A 2014, 2, 6107–6114. [Google Scholar] [CrossRef] [Green Version]
  100. Downie, R.A.; Popuri, S.R.; Ning, H.; Reece, M.J.; Bos, J.-W.G. Effect of spark plasma sintering on the structure and properties of Ti1−xZrxNiSn half-Heusler alloys. Materials 2014, 7, 7094–7104. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  101. Fu, C.; Zhu, T.; Pei, Y.; Xie, H.; Wang, H.; Snyder, G.J.; Liu, Y.; Liu, Y.; Zhao, X. High band degeneracy contributes to high thermoelectric performance in p-type half-Heusler compounds. Adv. Energy Mater. 2014, 4, 1400600. [Google Scholar] [CrossRef]
  102. Gazka, K.; Populoh, S.; Xie, W.; Yoon, S.; Saucke, G.; Hulliger, J.; Weidenkaff, A. Improved thermoelectric performance of (Zr0.3Hf0.7)NiSn half-Heusler compounds by Ta substitution. J. Appl. Phys. 2014, 115, 183704. [Google Scholar] [CrossRef] [Green Version]
  103. Hsu, C.-C.; Liu, Y.-N.; Ma, H.-K. Effect of the Zr0.5Hf0.5CoSb1−xSnx/HfO2 half-Heusler nanocomposites on the ZT value. J. Alloys Compd. 2014, 597, 217–222. [Google Scholar] [CrossRef]
  104. Joshi, G.; He, R.; Engber, M.; Samsonidze, G.; Pantha, T.; Dahal, E.; Dahal, K.; Yang, J.; Lan, Y.; Kozinsky, B.; et al. NbFeSb-based p-type half-Heuslers for power generation applications. Energy Environ. Sci. 2014, 7, 4070–4076. [Google Scholar] [CrossRef]
  105. Krez, J.; Schmitt, J.; Snyder, G.J.; Felser, C.; Hermes, W.; Schwind, M. Optimization of the carrier concentration in phase-separated half-Heusler compounds. J. Mater. Chem. A 2014, 2, 13513–13518. [Google Scholar] [CrossRef] [Green Version]
  106. Misra, D.K.; Bhardwaj, A.; Singh, S. Enhanced thermoelectric performance of a new half-Heusler derivative Zr9Ni7Sn8 bulk nanocomposite: Enhanced electrical conductivity and low thermal conductivity. J. Mater. Chem. A 2014, 2, 11913–11921. [Google Scholar] [CrossRef] [Green Version]
  107. Rausch, E.; Balke, B.; Ouardi, S.; Felser, C. Enhanced thermoelectric performance in the p-type half-Heusler (Ti,Zr,Hf)CoSb0.8Sn0.2 system via phase separation. Phys. Chem. Chem. Phys. 2014, 16, 25258–25262. [Google Scholar] [CrossRef] [PubMed]
  108. Akram, R.; Zhang, Q.; Yang, D.; Zheng, Y.; Yan, Y.; Su, X.; Tang, X. Enhanced thermoelectric properties of La-doped ZrNiSn half-Heusler compound. J. Electron. Mater. 2015, 44, 3563–3570. [Google Scholar] [CrossRef]
  109. Appel, O.; Zilber, T.; Kalabukhov, S.; Beerib, O.; Gelbstein, Y. Morphological effects on the thermoelectric properties of Ti0.3Zr0.35Hf0.35Ni1+δSn alloys following phase separation. J. Mater. Chem. C 2015, 3, 11653–11659. [Google Scholar] [CrossRef]
  110. Chai, Y.W.; Oniki, T.; Kimura, Y. Microstructure and thermoelectric properties of a ZrNi1.1Sn half-Heusler alloy. Acta Mater. 2015, 85, 290–300. [Google Scholar] [CrossRef] [Green Version]
  111. Chen, L.; Gao, S.; Zeng, X.; Mehdizadeh Dehkordi, A.; Tritt, T.M.; Poon, S.J. Uncovering high thermoelectric figure of merit in (Hf,Zr)NiSn half-Heusler alloys. Appl. Phys. Lett. 2015, 107, 041902. [Google Scholar] [CrossRef] [Green Version]
  112. Fu, C.; Zhu, T.; Liu, Y.; Xie, H.; Zhao, X. Band engineering of high-performance p-type FeNbSb based half-Heusler thermoelectric materials for figure of merit ZT > 1. Energy Environ. Sci. 2015, 8, 216–220. [Google Scholar] [CrossRef]
  113. Fu, C.; Bai, S.; Liu, Y.; Tang, Y.; Chen, L.; Zhao, X.; Zhu, T. Realizing high figure of merit in heavy-band p-type half-Heusler thermoelectric materials. Nat. Commun. 2015, 6, 8144–8151. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  114. Hattori, K.; Miyazaki, H.; Yoshida, K.; Inukai, M.; Nishino, Y. Direct observation of the electronic structure in thermoelectric half-Heusler alloys Zr1−xMxNiSn (M = Y and Nb). J. Appl. Phys. 2015, 117, 205102. [Google Scholar] [CrossRef]
  115. He, R.; Kim, H.S.; Lan, Y.; Wang, D.; Chen, S.; Ren, Z. Investigating the thermoelectric properties of p-type half-Heusler Hfx(Zr,Ti)1−xCoSb0.8Sn0.2 by reducing Hf concentration for power generation. RSC Adv. 2015, 4, 64711–64716. [Google Scholar] [CrossRef]
  116. Huang, L.; He, R.; Chen, S.; Zhang, H.; Dahal, K.; Zhou, H.; Wang, H.; Zhang, Q.; Ren, Z. A new n-type half-Heusler thermoelectric material NbCoSb. Mater. Res. Bull. 2015, 70, 773–778. [Google Scholar] [CrossRef] [Green Version]
  117. Huang, L.; Wang, Y.; Shuai, J.; Zhang, H.; Yang, S.; Zhang, Q.; Ren, Z. Thermal conductivity reduction by isoelectronic elements V and Ta for partial substitution of Nb in half-Heusler Nb(1−x)/2V(1−x)/2TaxCoSb. RSC Adv. 2015, 5, 102469–102476. [Google Scholar] [CrossRef]
  118. Li, S.; Zhao, H.; Li, D.; Jin, S.; Gu, L. Synthesis and thermoelectric properties of half-Heusler alloy YNiBi. J. Appl. Phys. 2015, 117, 205101–205108. [Google Scholar] [CrossRef]
  119. Liu, Y.; Xie, H.; Fu, C.; Snyder, G.J.; Zhao, X.; Zhu, T. Demonstration of a phonon-glass electron-crystal strategy in (Hf,Zr)NiSn half-Heusler thermoelectric materials by alloying. J. Mater. Chem. A 2015, 3, 22716–22722. [Google Scholar] [CrossRef]
  120. Rausch, E.; Balke, B.; Stahlhofen, J.M.; Ouardi, S.; Burkhardt, U.; Felser, C. Fine tuning of thermoelectric performance in phase-separated half-Heusler compounds. J. Mater. Chem. C 2015, 3, 10409. [Google Scholar] [CrossRef]
  121. Rausch, E.; Balke, B.; Ouardi, S.; Felser, C. Long-term stability of (Ti,Zr,Hf)CoSb1−xSnx thermoelectric p-type half-Heusler compounds upon thermal cycling. Energy Technol. 2015, 3, 1217–1224. [Google Scholar] [CrossRef]
  122. Schmitt, J.; Gibbs, Z.M.; Snyder, G.J.; Felser, C. Resolving the true band gap of ZrNiSn half-Heusler thermoelectric materials. Sci. Rep. 2015, 4, 6888. [Google Scholar] [CrossRef] [Green Version]
  123. Xie, H.; Wang, H.; Fu, C.; Liu, Y.; Snyder, G.J.; Zha, X.; Zhu, T. The intrinsic disorder related alloy scattering in ZrNiSn half-Heusler thermoelectric materials. Nature Commun. Sci. Rep. 2014, 4, 6888. [Google Scholar] [CrossRef] [Green Version]
  124. Akram, R.; Yan, Y.; Yang, D.W.; She, X.Y.; Zheng, G.; Su, X.L.; Tang, X.F. Microstructure and thermoelectric properties of Sb doped Hf0.25Zr0.75NiSn half-Heusler compounds with improved carrier mobility. Intermetallics 2016, 74, 1–7. [Google Scholar] [CrossRef]
  125. Chai, Y.W.; Oniki, T.; Kenjo, T.; Kimura, Y. The effect of an isoelectronic Ti-Zr substitution on Heusler nanoprecipitation and the thermoelectric properties of a (Ti0.2Zr0.8)Ni1.1Sn half-Heusler alloy. J. Alloys Compd. 2016, 662, 566–577. [Google Scholar] [CrossRef] [Green Version]
  126. Chauhan, N.S.; Bhardwaj, A.; Senguttuvan, T.D.; Pant, R.P.; Mallik, R.C.; Misra, D.K. A synergistic combination of atomic scale structural engineering and panoscopic approach in p-type ZrCoSb-based half-Heusler thermoelectric materials for achieving high ZT. J. Mater. Chem. C 2016, 4, 5766–5778. [Google Scholar] [CrossRef]
  127. Chen, L.; Zeng, X.; Tritt, T.M.; Poon, S.J. Half-Heusler alloys for efficient thermoelectric power conversion. J. Electron. Mater. 2016, 45, 5554–5556. [Google Scholar] [CrossRef] [Green Version]
  128. Fu, C.; Wu, H.; Liu, Y.; He, J.; Zhao, X.; Zhu, T. Enhancing the figure of merit of heavy-band thermoelectric materials through hierarchical phonon scattering. Adv. Sci. 2016, 3, 1600035. [Google Scholar] [CrossRef]
  129. Gürth, M.; Rogl, G.; Romaka, V.V.; Grytsiv, A.; Bauer, E.; Rogl, P. Thermoelectric high ZT half-Heusler alloys Ti1−x−yZrxHfyNiSn. Acta Mater. 2016, 104, 210–222. [Google Scholar] [CrossRef]
  130. He, R.; Huang, L.; Wang, Y.; Samsonidze, G.; Kozinsky, B.; Zhang, Q.; Ren, Z. Enhanced thermoelectric properties of n-type NbCoSn half-Heusler by improving phase purity. Appl. Mater. 2016, 4, 104804. [Google Scholar] [CrossRef] [Green Version]
  131. He, R.; Kraemer, D.; Mao, J.; Zeng, L.; Jie, Q.; Lan, Y.; Li, C.; Shuai, J.; Kim, H.S.; Liu, Y.; et al. Achieving high power factor and output power density in p-type half-Heuslers Nb1−xTixFeSb. Proc. Natl. Acad. Soc. USA 2016, 113, 13576–13581. [Google Scholar] [CrossRef]
  132. Huang, L.; Zhang, Q.; Yuan, B.; Lai, X.; Yan, X.; Ren, Z. Recent progress in half-Heusler thermoelectric materials. Mater. Res. Bull. 2016, 76, 107–112. [Google Scholar] [CrossRef]
  133. Liu, Y.; Makongo, J.P.A.; Page, A.; Sahoo, P.; Uher, C.; Stokes, K.; Poudeu, P.F.P. Distribution of impurity states and charge transport in Zr0.25Hf0.75Ni1+xSn1−ySby nanocomposites. J. Solid State Chem. 2016, 234, 72–86. [Google Scholar] [CrossRef] [Green Version]
  134. Rogl, G.; Grytsiv, A.; Gürth, M.; Tavassoli, A.; Ebner, C.; Wünschek, A.; Puchegger, S.; Soprunyuk, V.; Schranz, W.; Bauer, E.; et al. Mechanical properties of half-Heusler alloys. Acta Mater. 2016, 107, 178–195. [Google Scholar] [CrossRef]
  135. Visconti, A.; Bernard-Granger, G.; Navone, C.; Leforestier, J.; Mingo, N. Microstructure investigations and thermoelectric properties of an n-type half-Heusler alloy sintered by spark plasma sintering. Scripta Mater. 2016, 123, 100–104. [Google Scholar] [CrossRef]
  136. Zhang, H.; Wang, Y.; Huang, L.; Chen, S.; Dahal, H.; Wang, D.; Ren, Z. Synthesis and thermoelectric properties of n-type half-Heusler compound VCoSb with valence electron count of 19. J. Alloys Compd. 2016, 654, 321–326. [Google Scholar] [CrossRef] [Green Version]
  137. Zhang, H.; Wang, Y.; Dahal, H.; Mao, J.; Huang, L.; Zhang, Q.; Ren, Z. Thermoelectric properties of n-type half-Heusler compounds (Hf0.25Zr0.75)1−xNbxNiSn. Acta Mater. 2016, 113, 41–47. [Google Scholar] [CrossRef] [Green Version]
  138. Berry, T.; Ouardi, S.; Fecher, G.H.; Balke, B.; Kreiner, G.; Auffermann, G.; Schnelle, W.; Felser, C. Improving thermoelectric performance of TiNiSn by mixing MnNiSb in the half-Heusler structure. Phys. Chem. Chem. Phys. 2017, 19, 1543–1550. [Google Scholar] [CrossRef] [Green Version]
  139. Berry, T.; Fu, C.; Auffermann, G.; Fecher, G.H.; Schnelle, W.; Serrano-Sanchez, F.; Yue, Y.; Liang, H.; Felser, C. Enhancing thermoelectric performance of TiNiSn half-Heusler compounds via modulation doping. Chem. Mater. 2017, 29, 7042–7048. [Google Scholar] [CrossRef]
  140. Chen, L.; Liu, Y.; He, J.; Tritt, T.M.; Poon, S.J. High thermoelectric figure of merit by resonant dopant in half-Heusler alloys. AIP Adv. 2017, 7, 065208. [Google Scholar] [CrossRef] [Green Version]
  141. He, R.; Zhu, H.; Sun, J.; Mao, J.; Reith, H.; Chen, S.; Schierning, G.; Nielsch, K.; Ren, Z. Improved thermoelectric performance of n-type half-Heusler MCo1−xNixSb (M = Hf, Zr). Mater. Today Phys. 2017, 1, 24–30. [Google Scholar] [CrossRef]
  142. Huang, L.; Zhang, Q.; Wang, Y.; He, R.; Shuai, J.; Zhang, J.; Wang, C.; Ren, Z. The effect of Sn doping on thermoelectric performance of n-type half-Heusler NbCoSb. Phys. Chem. Chem. Phys. 2017, 19, 25683–25690. [Google Scholar] [CrossRef]
  143. Kim, K.S.; Kim, Y.M.; Mun, H.; Kim, J.; Park, J.; Borisevich, A.Y.; Lee, K.H.; Kim, S.W. Direct observation of inherent atomic-scale defect disorders responsible for high performance Ti1−xHfxNiSn1−ySby half-Heusler thermoelectric alloys. Adv. Mater. 2017, 29, 170209. [Google Scholar]
  144. Lkhagvasuren, E.; Ouardi, S.; Fecher, G.H.; Auffermann, G.; Kreiner, G.; Schnelle, W.; Felser, C. Optimized thermoelectric performance of the n-type half-Heusler material TiNiSn by substitution and addition of Mn. AIP Adv. 2017, 7, 045010. [Google Scholar] [CrossRef] [Green Version]
  145. Mao, J.; Zhou, J.; Zhu, H.; Liu, Z.; Zhang, H.; He, R.; Chen, G.; Ren, Z. Thermoelectric properties of n type ZrNiPb-based half-Heuslers. Chem. Mater. 2017, 29, 867–872. [Google Scholar] [CrossRef]
  146. Rogl, G.; Sauerschnig, P.; Rykavets, Z.; Romaka, V.V.; Heinrich, P.; Hinterleitner, B.; Grytsiv, A.; Bauer, E.; Rogl, P. (V,Nb)-doped half Heusler alloys based on {Ti,Zr,Hf}NiSn with high ZT. Acta Mater. 2017, 131, 336–348. [Google Scholar] [CrossRef]
  147. Silpawilawan, W.; Kurosaki, K.; Ohishi, Y.; Muta, H.; Yamanaka, S. FeNbSb p-type half-Heusler compound: Beneficial thermomechanical properties and high temperature stability for thermoelectrics. J. Mater. Chem. C 2017, 5, 6677–6681. [Google Scholar] [CrossRef]
  148. Tavassoli, A.; Failamani, F.; Grytsiv, A.; Rogl, G.; Heinrich, P.; Müller, H.; Bauer, E.; Zehetbauer, M.; Rogl, P. On the half-Heusler compounds Nb1−x{Ti,Zr,Hf}xFeSb: Phase relations, thermoelectric properties at low and high temperature, and mechanical properties. Acta Mater. 2017, 135, 263–276. [Google Scholar] [CrossRef]
  149. Visconti, A.; Navone, C.; Leforestier, J.; Mingo, N.; Bernard-Grange, G. Influence of the addition of HfO2 particles on the thermoelectric properties of an n-type half-Heusler alloy sintered by spark plasma sintering. J. Alloys Compds. 2017, 709, 36–41. [Google Scholar] [CrossRef]
  150. Yu, J.; Fu, C.; Liu, Y.; Xia, K.; Aydemir, U.; Chasapis, T.C.; Snyder, G.J.; Zhao, X.; Zhu, T. Unique role of refractory Ta alloying in enhancing the figure of merit of NbFeSb thermoelectric materials. Adv. Energy Mater. 2017, 8, 1701313. [Google Scholar] [CrossRef]
  151. Zeier, W.G.; Anand, S.; Huang, L.; He, R.; Zhang, H.; Ren, Z.; Wolverton, C.; Snyder, G.J. Using the 18-electron rule to understand the nominal 19-electron half-Heusler NbCoSb with Nb vacancies. Chem. Mater. 2017, 29, 1210–1217. [Google Scholar] [CrossRef]
  152. Barczak, S.A.; Halpin, J.E.; Buckman, J.; Decourt, R.; Pollet, M.; Smith, R.I.; MacLaren, D.A.; Bos, J.-W.G. Grain-by-grain compositional variations and interstitial metals—A new route toward achieving high performance in half-Heusler thermoelectrics. ACS Appl. Mater. Interfaces 2018, 10, 4786–4793. [Google Scholar] [CrossRef] [Green Version]
  153. Chauhan, N.S.; Bathula, S.; Vishwakarma, A.; Bhardwaj, R.; Gahtori, B.; Srivastava, A.K.; Saravanan, M.; Dhar, A. A nanocomposite approach for enhancement of thermoelectric performance in hafnium-free half-Heuslers. Materialia 2018, 1, 168–174. [Google Scholar] [CrossRef]
  154. Chauhan, N.S.; Bathula, S.; Vishwakarma, A.; Bhardwaj, R.; Johari, K.K.; Gahtori, B.; Saravanan, M.; Dhar, A. Compositional tuning of ZrNiSn half-Heusler alloys: Thermoelectric characteristics and performance analysis. J. Phys. Chem. Solids 2018, 123, 105–112. [Google Scholar] [CrossRef]
  155. Chauhan, N.S.; Bathula, S.; Vishwakarma, A.; Bhardwaj, R.; Gahtori, B.; Kumar, A.; Dhar, A. Vanadium-doping-induced resonant energy levels for the enhancement of thermoelectric performance in Hf-free ZrNiSn half-Heusler alloys. ACS Appl. Energy Mater. 2018, 1, 757–764. [Google Scholar] [CrossRef]
  156. Chauhan, N.S.; Bathula, S.; Vishwakarma, A.; Bhardwaj, R.; Johari, K.K.; Gahtori, B.; Dhar, A. Facile fabrication of p- and n-type half-Heusler alloys with enhanced thermoelectric performance and low specific contact resistance employing spark plasma sintering. Mater. Lett. 2018, 228, 250–253. [Google Scholar] [CrossRef]
  157. Hu, C.; Xia, K.; Chen, X.; Zhao, X.; Zhu, T. Transport mechanisms and property optimization of p-type (Zr,Hf)CoSb half-Heusler thermoelectric materials. Mater. Today Phys. 2018, 7, 69–76. [Google Scholar] [CrossRef]
  158. Liu, Y.; Fu, C.; Xia, K.; Chen, X.; Zhao, X.; Zhu, T. Lanthanide contraction as a design factor for high-performance half-Heusler thermoelectric materials. Adv. Mater. 2018, 30, 1800881. [Google Scholar] [CrossRef]
  159. Mallick, M.M.; Vitt, S. Enhancing the thermoelectric performance of a p-type half-Heusler alloy, HfCoSb by incorporation of a band-matched chalcogenide, Cu2Te. J. Mater. Chem. A 2018, 6, 14709–14716. [Google Scholar] [CrossRef]
  160. Ren, W.; Zhu, H.; Zhu, Q.; Saparamadu, U.; He, R.; Liu, Z.; Mao, J.; Wang, C.; Nielsch, K.; Wang, Z.; et al. Ultrahigh power factor in thermoelectric system Nb0.95M0.05FeSb (M = Hf, Zr, and Ti). Adv. Sci. 2018, 5, 1800278. [Google Scholar] [CrossRef]
  161. Rogl, G.; Yubuta, K.; Romaka, V.V.; Michor, H.; Schafler, E.; Grytsiv, A.; Bauer, E.; Rogl, P. High-ZT half-Heusler thermoelectrics, Ti0.5Zr0.5NiSn and Ti0.5Zr0.5NiSn0.98Sb0.02: Physical properties at low temperatures. Acta Mater. 2018, 166, 466–483. [Google Scholar] [CrossRef]
  162. Shen, J.; Fu, C.; Liu, Y.; Zhao, X.; Zhu, T. Enhancing thermoelectric performance of FeNbSb half-Heusler compound by Hf-Ti dual doping. Energy Storage Mater. 2018, 10, 69–74. [Google Scholar] [CrossRef]
  163. Silpawilawan, W.; Ohishi, Y.; Muta, H.; Yamanaka, S.; Kurosaki, K. Thermoelectric properties of p-type half-Heusler compounds FeNb0.9M0.1Sb (M = Ti, Zr, Hf). Mater. Trans. 2018, 59, 1030–1034. [Google Scholar] [CrossRef] [Green Version]
  164. Tavassoli, A.; Grytsiv, A.; Rogl, G.; Romaka, V.V.; Michor, H.; Reissner, M.; Bauer, E.; Zehetbauer, M.; Rogl, P. The half- Heusler system Ti1+xFe1.33−xSb—TiCoSb with Sb/Sn substitution: Phase relations, crystal structures and thermoelectric properties. Dalton Trans. 2018, 47, 879–897. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  165. Xia, K.; Liu, Y.; Anand, S.; Snyder, G.J.; Xin, J.; Yu, J.; Zhao, X.; Zhu, T. Enhanced thermoelectric performance in 18-electron Nb0.8CoSb half-Heusler compound with intrinsic Nb vacancies. Adv. Funct. Mater. 2018, 28, 1705845. [Google Scholar] [CrossRef]
  166. Yan, Y.; Geng, W.; Qiu, J.; Ke, H.; Luo, C.; Yang, J.; Uher, C.; Tang, X. Thermoelectric properties of n-type ZrNiSn prepared by rapid non-equilibrium laser processing. RSC Adv. 2018, 8, 15796–15803. [Google Scholar] [CrossRef]
  167. Zhao, D.; Zou, M.; Bo, L.; Wang, Y. Synthesis and thermoelectric properties of Pd-doped ZrCoBi half-Heusler compounds. Materials 2018, 11, 728–737. [Google Scholar] [CrossRef] [Green Version]
  168. Zhu, T.; Fu, C.; Zhao, X. High figure of merit p-type FeNbHfSb thermoelectric material and the preparation method thereof. United States Patent US2018/0331268A1, 15 November 2018. [Google Scholar]
  169. Zhu, H.; He, R.; Mao, J.; Zhu, Q.; Li, C.; Sun, J.; Ren, W.; Wang, Y.; Liu, Z.; Tang, Z.; et al. Discovery of ZrCoBi based half-Heuslers with high thermoelectric conversion efficiency. Nat. Commun. 2018, 9, 2497. [Google Scholar] [CrossRef]
  170. Barczak, S.A.; Quinn, R.J.; Halpin, J.E.; Domosud, K.; Smith, R.I.; Baker, A.R.; Don, E.; Forbes, I.; Refson, K.; MacLaren, D.A.; et al. Suppression of thermal conductivity without impeding electron mobility in n-type XNiSn half-Heusler thermoelectrics. J. Mater. Chem. A 2019, 7, 27124–27134. [Google Scholar] [CrossRef] [Green Version]
  171. Chauhan, N.S.; Bathula, S.; Gahtori, B.; Mahanti, S.D.; Bhattacharya, A.; Vishwakarma, A.; Bhardwaj, R.; Singh, V.N.; Dhar, A. Compositional tailoring for realizing high thermoelectric performance in hafnium-free n type ZrNiSn half-Heusler alloys. ACS Appl. Mater. Interfaces 2019, 11, 47830–47836. [Google Scholar] [CrossRef]
  172. Du, N.V.; Rahman, J.U.; Meang, E.-J.; Lim, C.-H.; Shin, W.H.; Seo, W.-S.; Huy, P.T.; Kim, M.H.; Lee, S. Synthesis and thermoelectric properties of Ti-substituted (Hf0.5Zr0.5)1−xTixNiSn0.998Sb0.002 half-Heusler compounds. J. Alloys Compd. 2019, 773, 1141–1145. [Google Scholar]
  173. Ferluccio, D.A.; Halpin, J.E.; MacIntosh, K.L.; Quinn, R.J.; Don, E.; Smith, R.I.; MacLaren, D.A.; Bos, J.-W.G. Low thermal conductivity and promising thermoelectric performance in AxCoSb (A = V, Nb or Ta) half-Heuslers with inherent vacancies. J. Mater. Chem. C 2019, 7, 6539–6547. [Google Scholar] [CrossRef] [Green Version]
  174. Gong, B.; Li, Y.; Liu, F.; Zhu, J.; Wang, X.; Ao, W.; Zhang, C.; Li, J.; Xie, H.; Zhu, T. Continuously enhanced structural disorder to suppress the lattice thermal conductivity of ZrNiSn-based half-Heusler alloys by multielement and multisite alloying with very low Hf content. ACS Appl. Mater. Interfaces 2019, 11, 13397–13404. [Google Scholar] [CrossRef] [PubMed]
  175. Grytsiv, A.; Romaka, V.V.; Watson, N.; Rogl, G.; Michor, H.; Hinterleitner, B.; Puchegger, S.; Bauer, E.; Rogl, P. Thermoelectric half-Heusler compounds TaFeSb and Ta1−xTixFeSb (0 ≤ x ≤ 0.11): Formation and physical properties. Intermetallics 2019, 111, 106468. [Google Scholar] [CrossRef]
  176. Huang, L.; Wang, J.; Mo, X.; Lei, X.; Ma, S.; Wang, C.; Zhang, Q. Improving the thermoelectric properties of the half-Heusler compound VCoSb by vanadium vacancy. Materials 2019, 12, 1637. [Google Scholar] [CrossRef] [Green Version]
  177. Joshi, H.; Rai, D.P.; Hnamte, L.; Laref, A.; Thapa, R.K. A theoretical analysis of elastic and optical properties of half-Heusler MCoSb (M = Ti, Zr and Hf). Heliyon 2019, 5, e01155. [Google Scholar] [CrossRef] [Green Version]
  178. Karati, A.; Mukherjee, S.; Mallik, R.C.; Shabadi, R.; Murty, B.S.; Varadaraju, U.V. Simultaneous increase in thermopower and electrical conductivity through Ta-doping and nanostructuring in half-Heusler TiNiSn alloys. Materialia 2019, 7, 100410. [Google Scholar] [CrossRef]
  179. Karati, A.; Nagini, M.; Ghosh, S.; Shabadi, R.; Pradeep, K.G.; Mallik, R.C.; Murty, B.S.; Varadaraju, U.V. Ti2NiCoSnSb—A new half-Heusler type high-entropy alloy showing simultaneous increase in Seebeck coefficient and electrical conductivity for thermoelectric applications. Sci. Rep. 2019, 9, 5331–5343. [Google Scholar] [CrossRef] [Green Version]
  180. Li, S.; Zhu, H.; Mao, J.; Feng, Z.; Li, X.; Chen, C.; Cao, F.; Liu, X.; Singh, D.J.; Ren, Z.; et al. N-type TaCoSn-based half-Heuslers as promising thermoelectric materials. ACS Appl. Mater. Interfaces 2019, 11, 41321–41329. [Google Scholar] [CrossRef]
  181. Mallik, M.M.; Rajput, K.; Vitta, S. Increasing figure-of-merit of ZrNiSn half-Heusler alloy by minimal substitution and thermal conductivity reduction. J. Mater. Sci. Mater. Electron. 2019, 30, 6139–6147. [Google Scholar] [CrossRef]
  182. Qiu, Q.; Liu, Y.; Xia, K.; Fang, T.; Yu, J.; Zhao, X.; Zhu, T. Grain boundary scattering of charge transport in n-type (Hf,Zr)CoSb half-Heusler thermoelectric materials. Adv. Energy Mater. 2019, 9, 1803447. [Google Scholar] [CrossRef]
  183. Shen, J.; Wang, Z.; Chu, J.; Bai, S.; Zha, X.; Chen, L.; Zhu, T. Low contact resistivity and interfacial behavior of p-type NbFeSb/Mo thermoelectric junction. ACS Appl. Mater. Interfaces 2019, 11, 14182–14190. [Google Scholar] [CrossRef] [PubMed]
  184. Shen, J.; Fan, L.; Hu, C.; Zhu, T.; Xin, J.; Fu, T.; Zhao, D.; Zhao, X. Enhanced thermoelectric performance in the n-type NbFeSb half-Heusler compound with heavy element Ir doping. Mater. Today Phys. 2019, 8, 62–70. [Google Scholar] [CrossRef]
  185. Synoradzki, K.; Ciesielski, K.; Veremchuk, I.; Borrmann, H.; Skokowski, P.; Szymanski, D.; Grin, Y.; Kaczorowski, D. Thermal and electronic transport properties of the half-Heusler phase ScNiSb. Materials 2019, 12, 1723–1734. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  186. Xia, K.; Nan, P.; Tan, S.; Wang, Y.; Ge, B.; Zhang, W.; Anand, S.; Zhao, X.; Snyder, G.J.; Zhu, T. Short-range order in defective half-Heusler thermoelectric crystals. Energy Environ. Sci. 2019, 12, 1568–1574. [Google Scholar] [CrossRef]
  187. Zhu, H.; Mao, J.; Feng, Z.; Sun, J.; Zhu, Q.; Liu, Z.; Singh, D.J.; Wang, Y.; Ren, Z. Understanding the asymmetrical thermoelectric performance for discovering promising thermoelectric materials. Sci. Adv. 2019, 5, eaav5813. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  188. Zhu, H.; Mao, J.; Li, Y.; Sun, J.; Wang, Y.; Zhu, Q.; Li, G.; Song, Q.; Zhou, J.; Fu, Y.; et al. Discovery of TaFeSb-based half-Heuslers with high thermoelectric performance. Nat. Commun. 2019, 10, 270–278. [Google Scholar] [CrossRef] [Green Version]
  189. Bae, W.; Hwang, J.Y.; Kim, S.-I.; Jeong, H.M.; Kim, S.; Lim, J.-H.; Kim, H.S.; Lee, K.H. Thermoelectric transport properties of n-Type Sb-doped (Hf,Zr,Ti)NiSn half-Heusler alloys prepared by temperature-regulated melt spinning and spark plasma sintering. Appl. Sci. 2020, 10, 4963–4970. [Google Scholar] [CrossRef]
  190. Chauhan, N.S.; Raghuvanshi, P.R.; Tyagi, K.; Johari, K.K.; Tyagi, L.; Gahtori, B.; Bathula, S.; Bhattacharya, A.; Mahanti, S.D.; Singh, V.N.; et al. Defect engineering for enhancement of thermoelectric performance of (Zr,Hf)NiSn-based n-type half-Heusler alloys. J. Phys. Chem. C 2020, 124, 8584–8593. [Google Scholar] [CrossRef]
  191. Ciesielski, K.; Synoradzki, K.; Veremchuk, I.; Skokowski, P.; Szymański, D.; Grin, Y.; Kaczorowski, D. Thermoelectric performance of the half-Heusler phases RNiSb (R = Sc, Dy, Er, Tm, Lu): High mobility ratio between majority and minority charge carriers. Phys. Rev. Appl. 2020, 14, 054046. [Google Scholar] [CrossRef]
  192. El-Khouly, A.; Novitskii, A.; Adam, A.M.; Sedegov, A.; Kalugina, A.; Pankratova, D.; Karpenkov, D.; Khovaylo, V. Transport and thermoelectric properties of Hf-doped FeVSb half-Heusler alloys. J. Alloys Compd. 2020, 820, 153413. [Google Scholar] [CrossRef]
  193. El-Khouly, A.; Novitskii, A.; Serhiienko, I.; Kalugina, A.; Sedegov, A.; Karpenkov, D.; Voronin, A.; Khovaylo, V.; Adam, A.M. Optimizing the thermoelectric performance of FeVSb half-Heusler compound via Hf-Ti double doping. J. Power Sources 2020, 477, 228768. [Google Scholar] [CrossRef]
  194. Fang, T.; Xia, K.; Nan, P.; Ge, B.; Zhao, X.; Zhu, T. A new defective 19-electron TiPtSb half-Heusler thermoelectric compound with heavy band and low lattice thermal conductivity. Mater. Today Phys. 2020, 13, 100200. [Google Scholar] [CrossRef]
  195. Graziosi, P.; Kumarasinghe, C.; Neophytou, N. Material descriptors for the discovery of efficient thermoelectrics. ACS Appl. Energy Mater. 2020, 3, 5913–5926. [Google Scholar] [CrossRef]
  196. Johari, K.K.; Bhardwaj, R.; Chauhan, N.S.; Gahtori, B.; Bathula, S.; Auluck, S.; Dhakate, S.R. Band structure modification and mass fluctuation effects of isoelectronic germanium-doping on thermoelectric properties of ZrNiSn. ACS Appl. Mater. 2020, 3, 1349–1357. [Google Scholar] [CrossRef]
  197. Kang, H.B.; Poudel, B.; Li, W.; Lee, H.; Saparamadu, U.; Nozariasbmarz, A.; Kang, M.G.; Gupta, A.; Heremans, J.; Priy, S. Decoupled phononic-electronic transport in multi-phase n-type half-Heusler nanocomposites enabling efficient high temperature power generation. Mater. Today 2020, 36, 63–72. [Google Scholar] [CrossRef]
  198. Karati, A.; Hariharan, V.S.; Ghosh, S.; Prasad, A.; Nagini, M.; Guruvidyathri, K.; Mallik, R.C.; Shabadi, R.; Bichler, L.; Murty, B.S.; et al. Thermoelectric properties of half-Heusler high-entropy Ti2NiCoSn1−xSb1+x (x = 0.5, 1) alloys with VEC > 18. Scripta Mater. 2020, 186, 375–380. [Google Scholar] [CrossRef]
  199. Li, S.; Bai, F.; Wang, R.; Chen, C.; Li, X.; Cao, F.; Sui, J.; Liu, X.; Ren, Z.; Zhang, Q. Titanium doping to enhance thermoelectric performance of 19-electron VCoSb half-Heusler compounds with vanadium vacancies. Ann. Phys. 2020, 525, 1900440. [Google Scholar] [CrossRef]
  200. Li, N.; Zhu, H.; He, W.; Zhang, B.; Cui, W.; Hu, Z.-Y.; Sang, X.; Lu, X.; Wang, G.; Zhou, X. Realizing both n- and p-types of high thermoelectric performance in Fe1-xNixTiSb half-Heusler compounds. J. Mater. Chem. C 2020, 8, 3156–3164. [Google Scholar] [CrossRef]
  201. Pedersen, S.V.; Croteau, J.R.; Kempf, N.; Zhang, Y.; Butt, D.P.; Jaques, B.J. Novel synthesis and processing effects on the figure of merit for NbCoSn, NbFeSb, and ZrNiSn based half-Heusler thermoelectrics. J. Solid State Chem. 2020, 285, 121203. [Google Scholar] [CrossRef]
  202. Serrano-Sanchez, F.; Luo, T.; Yu, J.; Xie, W.; Le, C.; Auffermann, G.; Weidenkaff, A.; Zhu, T.; Zhao, X.; Alonso, J.A.; et al. Thermoelectric properties of n-type half-Heusler NbCoSn with heavy-element Pt substitution. J. Mater. Chem. A 2020, 8, 14822–14828. [Google Scholar] [CrossRef]
  203. Yang, X.; Jiang, Z.; Kang, H.; Chen, Z.; Guo, E.; Liu, D.; Yang, F.; Li, R.; Jiang, X.; Wang, T. Enhanced thermoelectric performance of Zr1−xTaxNiSn half-Heusler alloys by diagonal-rule doping. ACS Appl. Mater. Interfaces 2020, 12, 3773–3783. [Google Scholar] [CrossRef] [PubMed]
  204. Yu, J.; Xing, Y.; Hu, C.; Huang, Z.; Qiu, Q.; Wang, C.; Xia, K.; Wang, Z.; Bai, S.; Zhao, X.; et al. Half-Heusler thermoelectric module with high conversion efficiency and high-power density. Adv. Energy Mater. 2020, 10, 2000888. [Google Scholar] [CrossRef]
  205. Wang, R.-F.; Li, S.; Xue, W.-H.; Chen, C.; Wang, Y.-M.; Liu, X.-J.; Zhang, Q. Enhanced thermoelectric performance of n-type TiCoSb half- Heusler by Ta doping and Hf alloying. Rare Met. 2021, 40, 40–47. [Google Scholar] [CrossRef]
  206. Zelenka, F.; Brož, P.; Vrestal, J.; Bursik, J.; Zemanova, A.; Rogl, G.; Rogl, P. Study of thermal stability of half-Heusler alloys TiFe1.33Sb and TixNb1−xFeSb (x = 0, 0.15) by differential thermal analysis and Knudsen effusion method. Calphad 2021, 74, 102292. [Google Scholar] [CrossRef]
  207. Guo, S.; Anad, S.; Brod, M.K.; Zhang, Y.; Snyder, G.J. Conduction band engineering of half-Heusler thermoelectrics using orbital chemistry. J. Mater. Chem. A 2022, 10, 3051. [Google Scholar] [CrossRef]
  208. Karati, A.; Ghosh, S.; Nagini, M.; Mallik, R.C.; Shabadi, R.; Murty, B.S.; Varadaraju, U.V. Thermoelectric properties of nanocrystalline half-Heusler high-entropy Ti2NiCoSn1−xSb1+x(x = 0.3, 0.5, 0.7, 1) alloys with VEC > 18. J. Alloys Compds. 2022, 927, 166578. [Google Scholar] [CrossRef]
  209. Karati, A.; Mishra, S.R.; Ghosh, S.; Mallik, R.C.; Shabadi, R.; Murty, B.S.; Ramanujan, R.V.; Yadav, S.K.; Murty, B.S.; Varadaraju, U.V. Thermoelectric properties of a high entropy half-Heusler alloy processed by a fast powder metallurgy route. J. Alloys Compd. 2022, 924, 166108. [Google Scholar] [CrossRef]
  210. Mesaritis, G.; Ioannou, I.; Delimitis, A.; Hatzikraniotis, E.; Gelbstein, Y.; Kyratsi, T. n-type (Zr,Ti)NiSn half-Heusler materials via mechanical alloying: Structure, Sb-doping and thermoelectric properties. J. Phys. Chem. Solids 2022, 167, 110735. [Google Scholar] [CrossRef]
  211. Serrano-Sanchez, F.; Yao, M.; He, B.; Chen, D.; Gloskovskii, A.; Fedorov, A.; Auffermann, G.; Liu, E.; Burkhardt, U.; Fecher, G.H.; et al. Electronic structure and low-temperature thermoelectric transport of TiCoSb single crystals. Nanoscale 2022, 14, 10067. [Google Scholar] [CrossRef]
  212. Yang, X.; Wang, Y.; Min, R.; Chen, Z.; Guo, E.; Kang, H.; Li, L.; Jiang, X.; Wang, T. Enhancement in thermoelectric properties of ZrNiSn-based alloys by Ta-Hf Co-doping. Acta Mater. 2022, 233, 117976. [Google Scholar] [CrossRef]
  213. Barczak, S.A.; Kennedy, B.F.; da Silva, I.; Bos, J.-W.G. Mechanistic insights into the formation of thermoelectric TiNiSn from in situ neutron powder diffraction. Chem. Mater. 2023, 35, 3694. [Google Scholar] [CrossRef] [PubMed]
  214. Chen, R.; Yan, Y.; Zhang, W.; Liu, F.; Kang, H.; Guo, E.; Chen, Z.; Wang, T. Enhanced thermoelectric properties of Zr0.85−xHfxNb0.15−yTayCoSb medium-entropy alloys: Tradeoff between “What to Alloy” and “How Much to Alloy”. Chem. Mater. 2023, 35, 2202. [Google Scholar] [CrossRef]
  215. Ciesielski, K.; Synoradzki, K.; Szymański, D.; Tobita, K.; Berent, K.; Obstarczyk, P.; Kimura, K.; Kaczorowski, D. Half- Heusler phase TmNiSb under pressure: Intrinsic phase separation, thermoelectric performance and structural transition. Sci. Rep. 2023, 13, 1592. [Google Scholar] [CrossRef]
  216. Liu, R.; Wang, C.; Huang, J.; Luo, P.; Liu, X.; Ye, S.; Dong, Z.; Zhang, J.; Luo, J. Preparation and thermoelectric properties of Sc-doped Ti1−xNiSb half-Heusler alloys. Acta Phys. Sin. 2013, 72, 20230035. [Google Scholar]
  217. Min, R.; Gao, Y.; Jiang, X.; Yang, X.; Li, L.; Kang, H.; Guo, E.; Chen, Z.; Wang, T. Enhancement in the thermoelectric performance of ZrNiSn-based alloys through extra Zr-rich nanoprecipitates with superstructures. Chem. Eng. J. 2023, 464, 142531. [Google Scholar] [CrossRef]
  218. Verma, A.K.; Johari, K.K.; Dubey, P.; Candolfi, C.; Lenoir, B.; Walia, S.; Dhakate, S.R.; Gahtori, B. Coupling of electronic transport and defect engineering substantially enhances the thermoelectric performance of p-type TiCoSb HH alloy. J. Alloys Compds. 2023, 947, 169416. [Google Scholar] [CrossRef]
  219. Vikram, B.S.; Barman, C.K.; Alam, A. Accelerated discovery of new 8-electron half-Heusler compound as promising energy and topological quantum materials. J. Phys. Chem. C 2019, 123, 12. [Google Scholar] [CrossRef]
  220. Vikram, B.S.; Kangsabanik, J.; Alam, A. Accurate high-throughput screening of I-II-V 8-electron half-Heusler compounds for renewable-energy application. arXiv 2019, arXiv:1910.02984v1. [Google Scholar]
  221. Hoat, D.M.; Naseri, M. Electronic and thermoelectric properties of RbYSn half-Heusler compound with a 8 valence electrons: Spin-orbit coupling effect. Chem. Phys. 2020, 528, 110510. [Google Scholar] [CrossRef]
Crystals 13 01152 g001 550
Figure 1. Peak ZT, ZTmax, of p- and n-type half-Heusler alloys vs. year of publication.
Figure 1. Peak ZT, ZTmax, of p- and n-type half-Heusler alloys vs. year of publication.
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Crystals 13 01152 g002 550
Figure 2. Peak ZT, ZTmax, of p- and n-type half-Heusler alloys vs. temperature, T.
Figure 2. Peak ZT, ZTmax, of p- and n-type half-Heusler alloys vs. temperature, T.
Crystals 13 01152 g002
Crystals 13 01152 g003 550
Figure 3. P- and n-type half-Heusler alloys: power factor, pf, vs. thermal conductivity, λ, at 300 K.
Figure 3. P- and n-type half-Heusler alloys: power factor, pf, vs. thermal conductivity, λ, at 300 K.
Crystals 13 01152 g003
Crystals 13 01152 g004 550
Figure 4. P- and n-type half-Heusler alloys: power factor, pf, vs. thermal conductivity, λ, at 800 K.
Figure 4. P- and n-type half-Heusler alloys: power factor, pf, vs. thermal conductivity, λ, at 800 K.
Crystals 13 01152 g004
Crystals 13 01152 g005 550
Figure 5. ZT at 300 K vs. electrical resistivity, ρ, for p- and n-type half-Heusler alloys. Insert: cutout for ρ < 15000 µΩcm.
Figure 5. ZT at 300 K vs. electrical resistivity, ρ, for p- and n-type half-Heusler alloys. Insert: cutout for ρ < 15000 µΩcm.
Crystals 13 01152 g005
Crystals 13 01152 g006 550
Figure 6. ZT at 800 K vs. electrical resistivity, ρ, for p- and n-type half-Heusler alloys. Insert: cutout for ρ < 12000 µΩcm.
Figure 6. ZT at 800 K vs. electrical resistivity, ρ, for p- and n-type half-Heusler alloys. Insert: cutout for ρ < 12000 µΩcm.
Crystals 13 01152 g006
Crystals 13 01152 g007 550
Figure 7. ZT at 300 K vs. Seebeck coefficient, S, for p- and n-type half-Heusler alloys.
Figure 7. ZT at 300 K vs. Seebeck coefficient, S, for p- and n-type half-Heusler alloys.
Crystals 13 01152 g007
Crystals 13 01152 g008 550
Figure 8. ZT at 800 K vs. Seebeck coefficient, S, for p- and n-type half-Heusler alloys.
Figure 8. ZT at 800 K vs. Seebeck coefficient, S, for p- and n-type half-Heusler alloys.
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Crystals 13 01152 g009 550
Figure 9. ZT at 800 K vs. thermal conductivity, λ, for p- and n-type half-Heusler alloys. Insert: ZT at 300 K vs. thermal conductivity, λ.
Figure 9. ZT at 800 K vs. thermal conductivity, λ, for p- and n-type half-Heusler alloys. Insert: ZT at 300 K vs. thermal conductivity, λ.
Crystals 13 01152 g009
Crystals 13 01152 g010 550
Figure 10. ZT at 800 K vs. lattice thermal conductivity, λph, for p- and n-type half-Heusler alloys. Insert: ZT at 300 K vs. lattice thermal conductivity, λph.
Figure 10. ZT at 800 K vs. lattice thermal conductivity, λph, for p- and n-type half-Heusler alloys. Insert: ZT at 300 K vs. lattice thermal conductivity, λph.
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Crystals 13 01152 g011 550
Figure 11. ZT at 300 K for the HH n-type (Ti,Zr,Hf)NiSn-system.
Figure 11. ZT at 300 K for the HH n-type (Ti,Zr,Hf)NiSn-system.
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Crystals 13 01152 g012 550
Figure 12. ZT at 800 K and ZTmax for the HH n-type (Ti,Zr,Hf)NiSn-system.
Figure 12. ZT at 800 K and ZTmax for the HH n-type (Ti,Zr,Hf)NiSn-system.
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Crystals 13 01152 g013 550
Figure 13. ZT at 300 K for the HH n-type (Ti,Zr,Hf)CoSb-system, (V,NbTa)FeSb-system, 19-electron system and other half-Heuslers.
Figure 13. ZT at 300 K for the HH n-type (Ti,Zr,Hf)CoSb-system, (V,NbTa)FeSb-system, 19-electron system and other half-Heuslers.
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Crystals 13 01152 g014 550
Figure 14. ZT at 800 K for the HH n-type (Ti,Zr,Hf)CoSb-system, (V,NbTa)FeSb-system, 19-electron system and other half-Heuslers.
Figure 14. ZT at 800 K for the HH n-type (Ti,Zr,Hf)CoSb-system, (V,NbTa)FeSb-system, 19-electron system and other half-Heuslers.
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Crystals 13 01152 g015 550
Figure 15. ZT at 300 K for the p-type HH alloys.
Figure 15. ZT at 300 K for the p-type HH alloys.
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Figure 16. ZT at 800 K and ZTmax for the p-type HH alloys.
Figure 16. ZT at 800 K and ZTmax for the p-type HH alloys.
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Table
Table 1. N-type half-Heusler alloys.
Table 1. N-type half-Heusler alloys.
N-Type Half-Heusler Alloys
(Ti,Zr,Hf)NiSn-System		XZReferences (Marked with *: Not Measured above RT)
TiNiSn [19,21,22,23], [31] *, [30,34,39,58,75,83,88,90,99,139,143,144,152,178,210,213]
TiNiSn/Z Sb[24,39,58]
Si[34]
(Ti,X)NiSnNb [66]
Ta [178,212]
Ti(Ni,X)SnMn [138,139,144]
Pd [17]
Pt [34]
Cu [152]
Ti(Ni,X)Sn/ZCoSb[48,67,179,198,207,208]
ZrNiSn [6,7,10,12,14,17], [29,31,32] *, [33,35,36,37,42,57,66,70,71,85,99,106,108,110,119,122,123,129,153,154,155,166,171,174,196,203,210,212]
ZrNiSn/Z In[32] *
Si[181]
Ge[181,196]
Sb[10,17,19], [32] *, [37,123,195]
Bi[145]
(Zr,X)NiSnSc [122]
Y [57,114]
V [155]
Nb [66]
Ta [203,212]
Zr(Ni,X)SnPd [33,34]
Cu [36]
Co + Cu [36]
Zr(Ni,X)Sn/ZPdSb[33]
HfNiSn [12], [31] *, [99,119,129,195]
HfNiSn/Z Sb[129]
(Ti,Zr)NiSn [25,26,27,31,34,65,69,90,99,100,129,161]
(Ti,Zr)NiSn/Z Sb[25,26,27,90,99,100,149,161,201,210]
(Ti,Zr,X)NiSn/ZVSb[146]
NbSb[146]
(Ti,Hf)NiSn [31] *, [34,58,65,69,90,102,124,129,134,143]
(Ti,Hf)NiSn/Z Sb[58,143]
(Ti,Hf,X)NiSnNb [31] *
(Ti,Hf)(Ni,X)SnPd [58]
(Zr,Hf)NiSn [11], [31,32] *, [65,77,78,80,81,84,87,90,93,99,109,137,154,156,190]
(Zr,Hf)NiSn/Z Sb[11,12,18,43,49,50,73,76,77,79,89,93,95,98,111,119,124,127,129,132,133,135,138,140,146,172,187,201,204]
(Zr,Hf,X)NiSnV [81]
Nb [31] *
Ta [203,212]
(Zr,Hf)(Ni,X)SnPd [17], [29] *, [33,49,50]
Pt [95]
(Zr,Hf,X)NiSn/ZYSb[73]
VSb[140]
NbSb[137,140]
TaSb[140]
(Zr,Hf)(Ni,X)Sn/ZPdSb[33]
(Ti,Zr,Hf)NiSn [40,41,43,47], [49] *, [105,129,146,170,183]
(Ti,Zr,Hf)NiSn/Z Sb[43,127,189]
Bi[77,93]
Te[93]
(Ti,Zr,Hf,X)NiSnNb [105]
(Ti,Zr,Hf)(Ni,X)SnCu [170]
(Ti,Zr,Hf,X)NiSn/ZV + NbSb[174]
(Ti,Zr)Ni1±xSn [22,110,171]
(Ti,Zr)Ni1±xSn [138]
(Zr,Hf)Ni1±xSn [77]
TiNiSn+full Heusler [83]
TiNiSn+HfO2 [149]
ZrNiSn+B [29] *
ZrNiSn+La [108]
ZrNiSn+ZrO2 [35]
ZrNiSn+ZnO [153]
(Zr,Hf)NiSn+W [197]
(Zr,Hf)NiSn+ZrO2 [95]
(Zr,Hf)NiSn/Z+Nb Sb[80] *
(Zr,Hf)NiSn/Z+full Heusler Sb[77,78]
(Zr,Hf)(Ni,X)Sn/Z+ZrO2PdSb[49]
(Ti,Zr,Hf)NiSn/Z+ZrO2 Sb[17]
(Ti,Zr,Hf)CoSb-System
TiCoSb [38,44,46,48,55,56,61,68,177]
(Ti,X)CoSbTa [204]
Ti(Co,X)SbFe [61]
Ni [48]
(Ti,X)CoSb/ZNb + TaSn[38]
ZrCoSb [3,45,46,56,60,141,147,177]
(Zr,X)CoSbNb [156]
Zr(Co,X)SbNi [141]
HfCoSb [45,46,56,177,195]
(Hf,X)CoSbNb [182]
(Ti,Zr)CoSb [67]
(Ti,Zr)(Co,X)SbNi [67]
(Ti,Hf,X)CoSbTa [204]
(Zr,Hf,X)CoSbNb [158,182]
Nb + Ta [214]
(Ti,Zr,Hf)CoSb [56,64]
(Ti,Zr,Hf)(Co,X)SbNi [64]
(V,Nb,Ta)FeSb-System
VFeSb [74,91,192]
(V,X)FeSbTi [15]
NbFeSb [26,148]
Nb(Fe,X)SbIr [184]
(V,Nb)FeSb [92]
(V,Nb)(Fe,X)SbCo [92]
Ti(Fe,X)SbNi [200]
Ti(Fe,X)Sb/ZNiSn[13]
19-Electron System
VCoSb [136,173,176,199]
VCoSb/Z Sn[136]
NbCoSb [116,117,142,151,165,173]
Nb(Co,X)SbNi [186]
NbCoSb/Z Sn[142]
TaCoSb [173]
(Ti,V)CoSb [136,199]
(V,Nb)CoSb [117]
(Nb,Ta)CoSb [117]
(V,Nb,Ta)CoSb [117]
(Ti,Nb,Ta)CoSb/Z Sn[38]
TiNiSb [212]
(Ti,X)NiSbSc [212]
TiPtSn [194]
Other N-type Half-Heusler Material
TiCoSn [21]
TiPtSn [51]
VCoSn [207]
NbCoSn [130,180,202]
NbCoSn/Z Sb[52,63]
Nb(Co,X)SnPt [202]
(Nb,Ti)Sn/Z Sb[52]
TaCoSn [180]
(Ta,X)CoSnNb [180]
(Ta,X)CoSn/ZNbSb[180]
ZrCoBi [167,187]
Table
Table 2. P-type half-Heusler alloys.
Table 2. P-type half-Heusler alloys.
P-Type Half-Heusler Alloys
(Ti,Zr,Hf)NiSn-System		XZ References
(Zr,X)NiSnSc [122]
Y [114]
ZrNiSn + Co [71]
ZrNiSn + Ir [71]
(Ti,Zr,Hf)CoSb-System
TiCoSb [54,211]
TiCoSb/Z Ge [68]
TiCoSb/Z Sn [55,107,120,164]
ZrCoSb/Z Sn [16,19,60,107,126]
HfCoSb/Z Sn[107,120]
(Ti.Zr)CoSb/Z Sn [107]
(Ti,Hf)CoSb/Z Sn[18,86,107,120,121,132,159]
(Zr,Hf)CoSb/Z Sn[62,76,82,98,107,115,132,156,157]
(Ti,Zr,Hf)CoSb/Z Sn[96,107,121]
(Ti,Hf)CoSb/Z+Cu1.96Ni0.04Te0.97Se0.03 Sn[159]
(Zr,Hf)CoSb/Z+HfO2 Sn[103]
(Ti,Zr)[Fe,(Fe,Co), (Fe,Ni)]Sb-System
TiFeSb [164]
Ti(Fe,Co)Sb [61,94,164,206,218]
Ti(Fe,Co)Sb/Z Sn[164]
(Ti,Zr)(Co,Fe)Sb [218]
(Ti,Zr)(Co,Fe)Sb/Z Sn[218]
Ti(Fe,Ni)Sb [200]
Ti(Co,Fe)Sb+InSb [94]
(V,Nb,Ta)FeSb-System
(V,X)FeSbTi [15,97]
Hf [192]
Ti + Hf [193]
NbFeSb [27,112,131,147,195,206]
(Nb,X)FeSbTi [104,112,128,131,148,160,163,183,201,206]
Zr [113,148,160,163]
Hf [113,148,160,163,168]
Ti + Hf [26,162]
Ti + Ta [150,204]
(V,Nb,X)FeSbTi [101,112]
TaFeSb [175,188]
(Ta,X)FeSbTi [175,188]
Other P-Type Half-Heusler Material
NbCoSb/Z Sn[201]
ZrCoBi/Z Sn[169]
ZrPtSn [51]
HfPtSn [51]
ScNiSb [185,191]
DyNiSb [191]
ErNiSb [191]
TmNiSb [191,212]
LuNiSb [191]
YNiBi [118]
ErPdSb [53]
ErPdBi [53]
LaPdBi [59]
GdPdBi [59]
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Rogl, G.; Rogl, P.F. Development of Thermoelectric Half-Heusler Alloys over the Past 25 Years. Crystals 2023, 13, 1152. https://doi.org/10.3390/cryst13071152

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Rogl G, Rogl PF. Development of Thermoelectric Half-Heusler Alloys over the Past 25 Years. Crystals. 2023; 13(7):1152. https://doi.org/10.3390/cryst13071152

Chicago/Turabian Style

Rogl, Gerda, and Peter Franz Rogl. 2023. "Development of Thermoelectric Half-Heusler Alloys over the Past 25 Years" Crystals 13, no. 7: 1152. https://doi.org/10.3390/cryst13071152

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