Experimental Realization of Heavily p-doped Half-Heusler CoVSn Compound

Hypothetical half-Heusler (HH) ternary alloy of CoVSn has already been computationally investigated for possible spintronics and thermoelectric applications. We report the experimental realization of this compound and the characterizations of its thermoelectric properties. The material was synthesized by a solid-state reaction of the stoichiometric amounts of the elements via powder metallurgy (30 h mechanical milling and annealing at 900 ◦C for 20 h) and spark plasma sintering (SPS). The temperature-dependent ternary thermodynamic phase diagram of Co-V-Sn was further calculated. The phase diagram and detailed analysis of the synthesized material revealed the formation of the non-stoichiometry HH CoVSn, mixed with the binary intermetallic phases of SnV3, Co2Sn, and Co3V. The combination of X-ray diffraction, energy-dispersive X-ray spectroscopy, and thermoelectric transport properties confirmed the formation of a multi-phase compound. The analysis revealed the predicted thermoelectric features (zT = 0.53) of the highly doped CoVSn to be compromised by the formation of intermetallic phases (zT ≈ 0.007) during synthesis. The additional phases changed the properties from pto overall n-type thermoelectric characteristics.


Introduction
There have been substantial progress in thermoelectric (TE) materials over the last two decades. Thermoelectric technology, which was mainly based on alloys of bismuth telluride [1] for Peltier cooling modules, or silicon-germanium alloys [2] for radioisotope thermoelectric generators used in NASA spacecraft, has expanded to new compounds for power generation and cooling [3]. New materials and material structures have been discovered with considerably enhanced thermoelectric properties [4]. In particular, some materials like half-Heusler (HH) alloys have shown an inherently large thermoelectric power factor, although they have generally higher thermal conductivity than alloys such as Skutterudites [5] and Clathrates [6]. Since the thermal conductivity can be decreased by structural engineering, their potential to provide inherently significant power factors has attracted much attention lately. Recently, a new class of HH compounds was predicted with low thermal much attention lately. Recently, a new class of HH compounds was predicted with low thermal conductivity [7]. In this regard, the sheer number of HH compounds to investigate is vast, and computational and predictive methods to shortlist the promising ones have become essential to making quick progress. First principle studies [8] and thermodynamic calculations [9] are some of the main methods to predict the new materials. Notably, HH alloys with the crystal structure of C1b (No. 216) and the atomic arrangement of XYZ [10] have attracted much attention due to their unusual TE characteristics, high-temperature stability, and doping capabilities ( Figure 1, Table 1).

Figure-1.
Selected alloys with their corresponding TE properties, which have been studied as half-Heusler (numbers are the items given in Table 1). In 1995, Ö güt et al. [36] predicted CoVSn with a MgAgAs (C1b) crystal structure as an intermetallic semiconductor using density functional theory (DFT) band structure calculations. Another study [37] applied the full-potential linear muffin-tin orbital (FP-LMTO) method to evaluate the electronic properties of CoVSn alloy and showed an indirect energy bandgap of 0.75 eV. Shi et al. [38] calculated the electronic structure using the modified Becke-Johnson (MBJ) potential. Also, the transport coefficients were computed employing the Boltzmann theory within the constant scattering time approximation. Moreover, spin-orbit coupling (SOC) was considered in the electronic and transport calculations. Figure 2 shows the calculated electronic band structure and density of states (DOS) of CoVSn alloy [38]. Based on this electronic structure, the alloy was predicted as a p-type semiconductor with a bandgap (W-X) of 0.85 eV.  Table 1). Predicted TE factors of CoVSn compound, κ = 4.1 W/mK, S = 175 µV K −1 , zT = 0.53 at 900 K. [35] In 1995, Ögüt et al. [36] predicted CoVSn with a MgAgAs (C1b) crystal structure as an intermetallic semiconductor using density functional theory (DFT) band structure calculations. Another study [37] applied the full-potential linear muffin-tin orbital (FP-LMTO) method to evaluate the electronic properties of CoVSn alloy and showed an indirect energy bandgap of 0.75 eV. Shi et al. [38] calculated the electronic structure using the modified Becke-Johnson (MBJ) potential. Also, the transport coefficients were computed employing the Boltzmann theory within the constant scattering time approximation. Moreover, spin-orbit coupling (SOC) was considered in the electronic and transport calculations. Figure 2 shows the calculated electronic band structure and density of states (DOS) of CoVSn alloy [38]. Based on this electronic structure, the alloy was predicted as a p-type semiconductor with a bandgap (W-X) of 0.85 eV. In the mentioned study, the maximum Seebeck coefficient with a carrier concentration of 1.0 × 10 21 cm −3 was calculated at a temperature of 1150 K as 340 µ V/K [38]. Zeeshan et al. [39] carried out combined ab initio and semiclassical calculations based on Boltzmann transport theory and determined the maximum doping concentration of the p-type CoVSn alloy. Assuming a relaxation time of 10 −15 s for V-and Cr-group doping, they estimated a Seebeck coefficient of 175 µ V/K and a maximum power factor in the range of 11-23 µ W/K −2 at optimum 0.26 e/uc p-type doping. This value of the power factor is comparable to the power factor of many good HH thermoelectric alloys.
Despite the wide range of theoretical studies on CoVSn alloy with predicted promising thermoelectric characteristics, to the best of our knowledge, there is no report on the thermoelectric characterization of a synthesized sample of this alloy in the open literature. The current study presents an experimental synthesis and characterization of the CoVSn compound, and an empirical thermoelectric and microstructure analysis is performed.

Materials and Methods
The powder mixture of CoVSn compound was prepared by ball milling (SPEX-6, Metuchen, NJ, USA) (5:1 ball-powder weights) a stoichiometric ratio (1:1:1) of Co, V and Sn elements (Alfa Aesar Co., 99.9% commercial purity, Ward Hill, MA, USA) for 30 h following an annealing process at 900 °C for 20 h. The bulk samples were made by spark plasma sintering (SPS) at a temperature of 850 °C for 20 min at a pressure of 42 MPa. The phase identification was made via X-ray diffraction analysis (MiniFlex 300/600, 40 kV, 15 mA, Cu x-ray tube generation). The microstructure analysis was performed by field emission scanning electron microscopy (FESEM) (Quanta 450 FEG, FEI, Hillsboro, Oregon, USA). The thermal diffusivity (υ) was measured using a laser flash apparatus (LFA, Linseis, Selb, Germany) under an Ar environment from 30 to 550 °C. The Archimedes method was used to measure the density of materials with deionized water (DI) water as a displacement medium. Also, the specific heat was approximated by the Debye specific heat [40] for a temperature range of 30 to 600 °C. The electrical resistivity was measured using Linseis-LSR3 equipment (LSR3, Linseis, Selb, Germany) under He environment for the same temperature range. The Seebeck coefficient was measured simultaneously. The commercial Linsesis software calculates the thermopower from a single temperature gradient (∆T) and voltage difference (∆V), which is often erroneous. Therefore, the measurement was performed for five different temperature gradients, and each measurement was repeated four times, then averaged. The thermopower was calculated from the slope of the line fitted to five separate temperature and voltage differences. The accuracy of the measurement was verified by inspecting the linear fit to the (∆T-∆V) data set. Figure 3 shows the X-ray diffraction data of the CoVSn powder after 15 and 30 h milling, and that of the SPS-consolidated bulk sample prepared from the milled powder mixture that was annealed at 900 °C for 20 h. The XRD patterns of the SPS-consolidated CoVSn bulk sample revealed In the mentioned study, the maximum Seebeck coefficient with a carrier concentration of 1.0 × 10 21 cm −3 was calculated at a temperature of 1150 K as 340 µV/K [38]. Zeeshan et al. [39] carried out combined ab initio and semiclassical calculations based on Boltzmann transport theory and determined the maximum doping concentration of the p-type CoVSn alloy. Assuming a relaxation time of 10 −15 s for V-and Cr-group doping, they estimated a Seebeck coefficient of 175 µV/K and a maximum power factor in the range of 11-23 µW/K −2 at optimum 0.26 e/uc p-type doping. This value of the power factor is comparable to the power factor of many good HH thermoelectric alloys.

Results and Discussion
Despite the wide range of theoretical studies on CoVSn alloy with predicted promising thermoelectric characteristics, to the best of our knowledge, there is no report on the thermoelectric characterization of a synthesized sample of this alloy in the open literature. The current study presents an experimental synthesis and characterization of the CoVSn compound, and an empirical thermoelectric and microstructure analysis is performed.

Materials and Methods
The powder mixture of CoVSn compound was prepared by ball milling (SPEX-6, Metuchen, NJ, USA) (5:1 ball-powder weights) a stoichiometric ratio (1:1:1) of Co, V and Sn elements (Alfa Aesar Co., 99.9% commercial purity, Ward Hill, MA, USA) for 30 h following an annealing process at 900 • C for 20 h. The bulk samples were made by spark plasma sintering (SPS) at a temperature of 850 • C for 20 min at a pressure of 42 MPa. The phase identification was made via X-ray diffraction analysis (MiniFlex 300/600, 40 kV, 15 mA, Cu x-ray tube generation). The microstructure analysis was performed by field emission scanning electron microscopy (FESEM) (Quanta 450 FEG, FEI, Hillsboro, OR, USA). The thermal diffusivity (υ) was measured using a laser flash apparatus (LFA, Linseis, Selb, Germany) under an Ar environment from 30 to 550 • C. The Archimedes method was used to measure the density of materials with deionized water (DI) water as a displacement medium. Also, the specific heat was approximated by the Debye specific heat [40] for a temperature range of 30 to 600 • C. The electrical resistivity was measured using Linseis-LSR3 equipment (LSR3, Linseis, Selb, Germany) under He environment for the same temperature range. The Seebeck coefficient was measured simultaneously. The commercial Linsesis software calculates the thermopower from a single temperature gradient (∆T) and voltage difference (∆V), which is often erroneous. Therefore, the measurement was performed for five different temperature gradients, and each measurement was repeated four times, then averaged. The thermopower was calculated from the slope of the line fitted to five separate temperature and voltage differences. The accuracy of the measurement was verified by inspecting the linear fit to the (∆T-∆V) data set. Figure 3 shows the X-ray diffraction data of the CoVSn powder after 15 and 30 h milling, and that of the SPS-consolidated bulk sample prepared from the milled powder mixture that was annealed at 900 • C for 20 h. The XRD patterns of the SPS-consolidated CoVSn bulk sample revealed the presence Energies 2020, 13, 1459 4 of 11 of three binary intermetallics, namely, SnV 3 , Co 2 Sn, and Co 3 V, in addition to the non-stoichiometry CoVSn half-Heusler alloy. Therefore, the synthesized compound was shown to be a multi-phase material. Further, the back-scattered electron (BSE) image and the energy dispersive spectroscopy (EDS) maps of the material, as shown in Figure 4, illustrated a heterogeneous microstructure, which agrees with the observation from the XRD analysis. The non-uniform dispersion of Co, V, and Sn provide evidence of the presence of a multiphase structure with various micro-clusters.

Results and Discussion
Energies 2020, 13, x FOR PEER REVIEW 4 of 10 the presence of three binary intermetallics, namely, SnV3, Co2Sn, and Co3V, in addition to the nonstoichiometry CoVSn half-Heusler alloy. Therefore, the synthesized compound was shown to be a multi-phase material. Further, the back-scattered electron (BSE) image and the energy dispersive spectroscopy (EDS) maps of the material, as shown in Figure 4, illustrated a heterogeneous microstructure, which agrees with the observation from the XRD analysis. The non-uniform dispersion of Co, V, and Sn provide evidence of the presence of a multiphase structure with various micro-clusters.   Figure 5 shows the thermoelectric parameters S, σ, κ (thermal conductivity), and zT of the synthesized compound over a temperature range of 25 to 550 °C. The Seebeck coefficient was negative, indicating an n-type semiconductor. This is in contrast with the theoretical prediction that the CoVSn compound should be a p-type semiconductor [38,39]. The physical properties of the Heusler-based compound are generally highly related to the crystallographic order [41]. The observed multiphase structure ( Figure 5) combining metallic and semiconductor characteristics due to the presence of intermetallics (SnV3, Co2Sn, and Co3V) and a semiconductor (CoVSn), respectively. Therefore, the carrier type of the composition cannot be assigned to the single-phase half-Heusler CoVSn compound.  Energies 2020, 13, x FOR PEER REVIEW 4 of 10 the presence of three binary intermetallics, namely, SnV3, Co2Sn, and Co3V, in addition to the nonstoichiometry CoVSn half-Heusler alloy. Therefore, the synthesized compound was shown to be a multi-phase material. Further, the back-scattered electron (BSE) image and the energy dispersive spectroscopy (EDS) maps of the material, as shown in Figure 4, illustrated a heterogeneous microstructure, which agrees with the observation from the XRD analysis. The non-uniform dispersion of Co, V, and Sn provide evidence of the presence of a multiphase structure with various micro-clusters.   Figure 5 shows the thermoelectric parameters S, σ, κ (thermal conductivity), and zT of the synthesized compound over a temperature range of 25 to 550 °C. The Seebeck coefficient was negative, indicating an n-type semiconductor. This is in contrast with the theoretical prediction that the CoVSn compound should be a p-type semiconductor [38,39]. The physical properties of the Heusler-based compound are generally highly related to the crystallographic order [41]. The observed multiphase structure ( Figure 5) combining metallic and semiconductor characteristics due to the presence of intermetallics (SnV3, Co2Sn, and Co3V) and a semiconductor (CoVSn), respectively. Therefore, the carrier type of the composition cannot be assigned to the single-phase half-Heusler CoVSn compound.   Figure 5 shows the thermoelectric parameters S, σ, κ (thermal conductivity), and zT of the synthesized compound over a temperature range of 25 to 550 • C. The Seebeck coefficient was negative, indicating an n-type semiconductor. This is in contrast with the theoretical prediction that the CoVSn compound should be a p-type semiconductor [38,39]. The physical properties of the Heusler-based compound are generally highly related to the crystallographic order [41]. The observed multiphase structure ( Figure 5) combining metallic and semiconductor characteristics due to the presence of intermetallics (SnV 3 , Co 2 Sn, and Co 3 V) and a semiconductor (CoVSn), respectively. Therefore, the carrier type of the composition cannot be assigned to the single-phase half-Heusler CoVSn compound.

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As illustrated in Figure 4, the multi-phase structure containing various intermetallic compounds with both semiconductor and metallic characteristics provides semiconductor-metal interfaces [42]. The CoVSn compound containing the intermetallic phases has both ionic and covalent bonds. The presence of both ionic and covalent bonds in CoVSn (VSn: ionic-rock salt; CoSn: covalent-Zinc blend) can influence the stability of the semiconductor-metal interfaces (i.e., interface behavior) [43]. The metal-semiconductor contact lines up the chemical potentials and develops a Schottky barrier at the interface that can lead to distinct chemical and electrical properties different from the bulk compounds [44]. It is instructive to look at the thermodynamics of the ternary phase diagram of CoVSn. Such data are not currently available, being a new alloy. Therefore, we calculated the ternary phase diagram of the Co-V-Sn over the temperatures range of interest, 25 to 1200 • C. The Thermo-Calc 2016a package was used for this calculation. Figure 6 shows the ternary phase diagrams at the selected temperatures of 1100, 900, 600, and 25 • C. Table 2. Phase compositions of the nominated areas in Figure 6.   As illustrated in Figure 4, the multi-phase structure containing various intermetallic compounds with both semiconductor and metallic characteristics provides semiconductor-metal interfaces [42]. The CoVSn compound containing the intermetallic phases has both ionic and covalent bonds. The presence of both ionic and covalent bonds in CoVSn (VSn: ionic-rock salt; CoSn: covalent-Zinc blend) can influence the stability of the semiconductor-metal interfaces (i.e., interface behavior) [43]. The metal-semiconductor contact lines up the chemical potentials and develops a Schottky barrier at the interface that can lead to distinct chemical and electrical properties different from the bulk compounds [44]. It is instructive to look at the thermodynamics of the ternary phase diagram of CoVSn. Such data are not currently available, being a new alloy. Therefore, we calculated the ternary phase diagram of the Co-V-Sn over the temperatures range of interest, 25 to 1200 °C. The Thermo-Calc 2016a package was used for this calculation. Figure 6 shows the ternary phase diagrams at the selected temperatures of 1100, 900, 600, and 25 °C.  As illustrated in Figure 4, the multi-phase structure containing various intermetallic compounds with both semiconductor and metallic characteristics provides semiconductor-metal interfaces [42]. The CoVSn compound containing the intermetallic phases has both ionic and covalent bonds. The presence of both ionic and covalent bonds in CoVSn (VSn: ionic-rock salt; CoSn: covalent-Zinc blend) can influence the stability of the semiconductor-metal interfaces (i.e., interface behavior) [43]. The metal-semiconductor contact lines up the chemical potentials and develops a Schottky barrier at the interface that can lead to distinct chemical and electrical properties different from the bulk compounds [44]. It is instructive to look at the thermodynamics of the ternary phase diagram of CoVSn. Such data are not currently available, being a new alloy. Therefore, we calculated the ternary phase diagram of the Co-V-Sn over the temperatures range of interest, 25 to 1200 °C. The Thermo-Calc 2016a package was used for this calculation. Figure 6 shows the ternary phase diagrams at the selected temperatures of 1100, 900, 600, and 25 °C.

a b
Energies 2020, 13, x FOR PEER REVIEW 6 of 10  Table 2 give the phase compositions of the nominated areas in Figure 6. Table 2. Phase compositions of the nominated areas in Figure 6.
Energies 2020, 13, 1459 8 of 11 Table 3 lists the different phases at the center of the phase diagram (i.e., the equal atomic concentration of the elements over 100-degree temperature steps from 25 to 1100 • C). These diagrams further confirm that at thermodynamic equilibrium, the material decomposes into multiple phases, as listed in Table 3, which agrees with the observation in the microstructural analysis (Figures 3 and 4). Equilibrium line between two areas of (CoSn, BCC_B2 (Co,V,Sn)) and (ALTA_SIGMA (V, Co), CoSn, BCC_B2 (Co,V,Sn)) 900 Equilibrium line between two areas of (CoSn, BCC_B2 (Co,V,Sn)) and (LIQUID, CoSn, BCC_B2 (Co,V,Sn)) 1000 LIQUID, BCC_B2 (Co,V,Sn) 1100 LIQUID As shown in Table 3, there is no single phase of CoVSn, but mainly binary compounds of CoSn, SnV 3 , Co 3 V, and Sn 2 V 3 . Our observation of the multiphase structure, containing three binary intermetallics, contradicts previous studies [38,45]. In the theoretical analysis of this composition (1:1:1) [45], the calculated phase diagram showed a specific area for the stable CoVSn compound. However, the presence of this single-phase alloy was not experimentally confirmed.

Conclusions
The single-phase half-Heusler CoVSn was predicted theoretically as a stable thermodynamic material with prospective properties for spintronics and thermoelectric applications, although it was never experimentally confirmed. According to the theoretical and experimental data presented here, the CoVSn phase was found to be thermodynamically unstable, and its partial decomposition into metallic phases is unavoidable at the equilibrium state. In this study, the experimental realization of the CoVSn compound with a heterogeneous microstructure was represented. The material analysis showed the presence of the half-Heusler ternary alloy of non-stoichiometry CoVSn as the semiconductor phase, mixed with three binary intermetallics: SnV 3 , Co 2 Sn, and Co 3 V. The combination of X-ray diffraction, energy-dispersive X-ray spectroscopy, and transport properties confirmed the formation of the composite structure. The composite material demonstrated a metallic electronic behavior with a degenerate carrier concentration.