Ternary Aluminides of a New Homologous Series—CePt2Al2 and CePt3Al3: Crystal Structures and Thermal Properties

Murashova, Elena; Morozova, Yuliya; Dunaev, Sergey; Kurenbaeva, Zhanafiya; Tursina, Anna

doi:10.3390/cryst10060465

Open AccessArticle

Ternary Aluminides of a New Homologous Series—CePt₂Al₂ and CePt₃Al₃: Crystal Structures and Thermal Properties

Department of Chemistry, Moscow State University, 119991 Moscow, Russia

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Author to whom correspondence should be addressed.

Crystals 2020, 10(6), 465; https://doi.org/10.3390/cryst10060465

Submission received: 13 March 2020 / Revised: 21 May 2020 / Accepted: 22 May 2020 / Published: 1 June 2020

(This article belongs to the Special Issue Intermetallic)

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Abstract

:

In the process of studying the Ce–Pt–Al system, we identified CePt₂Al₂ and CePt₃Al₃, two new ternary intermetallic compounds. CePt₂Al₂ aluminide undergoes a structural phase transition from a low-temperature orthorhombic modification (of its own structure type, Cmme, a = 5.84138(2) Å, b = 6.39099(3) Å, c = 10.11611(5) Å) to a high-temperature tetragonal modification (CaBe₂Ge₂ type, P4/nmm, a = 4.3637(9) Å, c = 10.0925(14) Å) at 280(1) °C. CePt₃Al₃ crystallizes with a new type of structure (Cmme, a = 6.36548(6) Å, b = 5.78301(6) Å, c = 13.36245(19) Å) built of structural units of low-temperature orthorhombic CePt₂Al₂-type and CsCl-type.

Keywords:

intermetallic compounds; synthesis; crystal structure; phase transition

1. Introduction

The RET₂X₂ family (RE, rare earth element, actinoid, element of the 2nd group; T, transition metal; X, s- or p-block element) continues to attract attention from scientists due to their different physical properties related to strong electronic correlations such as heavy fermion states, superconductivity, valence fluctuations, unusual magnetic, and non-Fermi liquid behavior [1,2,3,4,5,6,7]. Most RET₂X₂ compounds crystallize in two structure types: CaBe₂Ge₂ (P4/nmm, a = 4.02(2) Å, c = 9.92(2) Å) [8] and ThCr₂Si₂ (I4/mmm, a = 4.043(1) Å, c = 10.577(2) Å) [9]. Both are ternary BaAl₄-type derivatives [10]. In the BaAl₄ structure type, Al atoms reside in two crystallographically different Wyckoff sites: 4d (0,1/2,1/4) and 4e (0,0,z). Those occupying 4d sites form two-dimensional square nets that are alternately capped above and below the plane by the atoms in 4e sites. Between the corrugated layers perpendicular to [001], Ba atoms are located.

The structure of ThCr₂Si₂ is an ordered version of BaAl₄ with more than 1700 ternary intermetallics being known as isotypic. In the ThCr₂Si₂ type_, 4d positions are filled by Cr, whereas those in 4e are occupied by Si atoms. Thus, the Cr atoms comprise the basal two-dimensional slab of square nets with Si atoms capping the nets in a “checkerboard” pattern. The corrugated [Cr₂Si₂] layers are inverted with respect to each other and are separated by Th atoms. The structure remains I-centered like the BaAl₄ prototype.

In the structure of CaBe₂Ge₂, which is not as rich in ternary intermetallics, filling the square nets of the basal slab and the capping layers occurs in an alternating manner. If the basal slab in the first layer is formed by Be atoms with Ge capping the square nets, in the next layer, Ge atoms build the basal slab that is capped by Be atoms. Due to this architecture of staggered [Be₂Ge₂] layers, CaBe₂Ge₂ has a primitive unit cell. Remarkably, CaBe₂Ge₂ type intermetallics are more likely to demonstrate superconductivity at high temperatures [11].

Intermetallics with platinum—REPt₂X₂ most commonly crystallize in the CaBe₂Ge₂ type. Silicides REPt₂Si₂ (RE = Y, La−Nd, Sm, Gd–Lu, U, Th) crystallize in a CaBe₂Ge₂ type and do not undergo phase transition [12], although it has been previously reported [13,14] that they belong to the ThCr₂Si₂ type with statistical filling of 4d and 4e positions with Pt and Si atoms in space group I4/mmm. Platinum germanides REPt₂Ge₂ (RE = Ca, Y, La−Dy) demonstrate a monoclinic variant (P2₁) of tetragonal CaBe₂Ge₂ structure with parameters for LaPt₂Ge₂ a = 4.401 Å, b = 4.421 Å, c = 9.851 Å, and β = 90.50° [15]. Reinvestigation of the structure showed a doubling of one of the parameters: a = 9.953 Å, b = 4.439 Å, c = 8.879 Å, β = 90.62°, and P2₁/c. The monoclinic cell undergoes a phase transition to a tetragonal type CaBe₂Ge₂ when the temperature is increased [16]. No phase transitions were observed in the CePt₂Sn₂ stannide, which belongs to the CaBe₂Ge₂ type [17].

Several pnictides with a REPt₂X₂ composition (X = P, As, Sb) with RE = Eu, Ca, Sr, Ba [18,19] adopt a CaBe₂Ge₂ type. REPt₂P₂ compounds where RE = Ca, Eu crystallize in a new structure type, a variation of the CaBe₂Ge₂ structure with a doubled c parameter and space group I4/mmm. The structures of SrPt₂Sb₂, BaPt₂As₂, and EuPt₂Sb₂ pnictides belong to the CaBe₂Ge₂ type, while the SrPt₂As₂ and EuPt₂As₂ arsenides present an orthorhombic distortion (Pmmn) of its tetragonal cell. Both compounds exhibit polymorphism: the orthorhombic modification of EuPt₂As₂ transforms to a tetragonal type CaBe₂Ge₂ with increasing temperatures, while SrPt₂As₂ undergoes phase transition to a monoclinic (P2₁/c) cell when pressure is increased.

About half of the 30 known aluminides RET₂Al₂ demonstrate a structure similar to CaBe₂Ge₂ including T = Au, RE = La−Nd, Sm, Eu, Gd−Dy, Th, U, and Sr [20] as well as T = Pd, RE = La, Ce [21]. The two latter compounds exhibit structural instability at low temperatures [22].

Recently, a homologous series structurally related to the title compounds was described [23]. Cerium palladium aluminides with the general formula CePd_nAl_n (n = 2–4) are built from CaBe₂Ge₂ and CsCl type structural fragments and crystallize in a tetragonal P4/nmm space group.

During our ongoing investigation of the Ce–Pt–Al phase diagram, two novel ternary aluminides were observed. Cerium platinum aluminum intermetallics CePt₂Al₂ and CePt₃Al₃ present a new homologous series CePt_nAl_n with n = 2, 3, derived from the orthorhombic CePt₂Al₂ and distorted CsCl type. Preliminary data on the crystal structures of the orthorhombic CePt₂Al₂ and CePt₃Al₃ have been presented at conferences [24,25]. Herein, we report on two structural modifications of CePt₂Al₂, tetragonal and orthorhombic, the structural phase transition between them as well as the crystal structure of CePt₃Al₃ and peculiarities of the homologous series CePt_nAl_n (n = 2, 3).

2. Materials and Methods

2.1. Synthesis

The synthesis of new compounds was performed using metallic cerium (99.98%), platinum (99.99%), and aluminum (99.999%) mixed in stoichiometric ratios by arc-melting in a pure argon atmosphere. In order to ensure homogenization, the alloys were overturned and melted several times. The ingot of CePt₂Al₂ was divided into six parts, sealed in evacuated quartz ampoules, and annealed at 250 °C, 320 °C, 550 °C, 650 °C, 700 °C, and 800 °C for 720 h. Afterward, the ampoules were rapidly quenched to room temperature using cold water. The alloy of CePt₃Al₃ was annealed in an evacuated ampoule at 700 °C for 720 h.

2.2. Energy Dispersive X-Ray Analysis

Energy dispersive X-ray (EDX) analysis of all annealed samples was performed using a Carl Zeiss LEO EVO 50XVP scanning electron microscope (SEM) with an EDX-spectrometer INCA Energy 450 (Oxford Instruments). The accelerating voltage was 20 kV. For quantitative microanalysis, the INCA energy dispersion microanalysis system contains predefined standards for all elements. Analysis accuracy can be improved by incorporating proprietary measured reference materials. CePtAl was used as an external standard. The samples under investigation were placed together with the standard in a hot pressing machine (Bühler), filled with an electrically conductive resin, and formed into a tablet. The surface of the tablet was sanded using sandpaper cloths of different grain sizes and then polished on a cloth with an Al₂O₃ paste. Finally, the tablet was washed for 5 min in an ultrasonic bath filled with ethanol. The uncertainty of measurements for each element did not exceed 0.7 at.%.

2.3. Powder X-Ray Diffraction

Powder X-ray diffraction (XRD) patterns for phase analysis and preliminary determination of unit cell parameters were obtained with a STOE STADI P transmission diffractometer (CuK_α1-radiation (λ = 1.54056 Å), Ge(111)-monochromator, a linear position-sensitive detector, 3–5° ≤ 2θ ≤ 93–95°, step scan 0.01°, 10 s counting time per point), using a WinXpow program [26].

2.4. High Temperature Powder Synchrotron X-Ray Diffraction

A high-intensity, high-resolution X-ray source (λ = 0.399962(13) Å) at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) was used in the temperature-dependent powder XRD experiments.

The powder of the sample was placed in an evacuated thin-walled quartz glass capillary with a diameter of 0.5 mm, which was rotated during measurements at a rate of 1200 rpm to improve the counting statistics. Calibration of the goniometer and refinement of the X-ray wavelength were performed using the Si NIST 640c silicon standard. Synchrotron XRD patterns were measured at an angle range of 2° ≤ 2θ ≤ 22.912° with a scan step of 0.002°.

2.5. Crystal Structure Determination

The crystal structures of the tetragonal and orthorhombic modifications of CePt₂Al₂ as well as CePt₃Al₃ were determined from experimental powder XRD data. Indexing of the powder XRD pattern was performed using TREOR and DICVOL programs implemented in WinXpow [26] and FULLPROFF [27,28] packages.

Tetragonal CePt₂Al₂. The preliminary parameters of tetragonal CePt₂Al₂ were established using a low-quality single crystal found as a single copy in a sample of stoichiometric composition heated to 1200 °C and quenched in ice-cold water, which allowed us to attribute the structure to the CaBe₂Ge₂ type.

The tetragonal unit cell parameters obtained from powder XRD (Table 1) were compatible with the structure types of ThCr₂Si₂ and CaBe₂Ge₂. Analysis of the systematic reflection conditions indicated a primitive unit cell, therefore the CaBe₂Ge₂ type was chosen as a structural model. For the Rietveld refinement of tetragonal CePt₂Al₂ with the MRIA program [29], a high-temperature powder XRD pattern collected at 350 °C was used. In the refinement, the observed anisotropic line broadening was approximated in the quartic form [30] with five variables in the case of tetragonal syngony. The result of the refinement is shown in Figure 1a and Table 1.

Orthorhombic CePt₂Al₂ and CePt₃Al₃. Careful examination of the systematic extinctions in the orthorhombic CePt₂Al₂ and CePt₃Al₃ datasets suggested a C-centered unit cell (h + k = 2n for all hkl), which prompted space groups Cmme, Cm2e, C2me, Cmm2, and C222. The best refinement results were obtained in the centrosymmetric space group Cmme. The structures of orthorhombic CePt₂Al₂ and CePt₃Al₃ were solved using the Patterson method with the SHELXS [31] program and sets of 115 and 162 reflection intensities, respectively, extracted from the powder XRD patterns after pseudo-Voigt fitting. The structures were refined via the Rietveld method using the FULLPROF program [27,28] for a single phase in the case of CePt₂Al₂ and for three phases in the case of CePt₃Al₃. For the latter, small impurities that had previously been detected by EDX (PtAl binary and orthorhombic CePt₂Al₂) were taken into account. The relevant crystallographic details for data collection and refinement are listed in Table 1; observed, calculated, and difference room-temperature XRD powder patterns are plotted in Figure 1b,c. The atomic coordinates and isotropic displacement parameters determined for tetragonal CePt₂Al₂, orthorhombic CePt₂Al₂, and CePt₃Al₃ are listed in Table 2, and selected interatomic distances are given in Table 3.

2.6. Differential Thermal Analysis

Thermal stability and temperature at which the structural phase transition of CePt₂Al₂ occurs were investigated by differential thermal analysis (DTA) at temperatures between 22 °C and 1200 °C, with a heating rate of 20° per minute in a stream of pure helium (sample mass ~20 mg) using a Netzsch STA449 F1 apparatus equipped with a Platinum RT analyzer.

3. Results and Discussion

3.1. Sample Characterization

Six samples of Ce_20.0Pt_40.0Al_40.0 (at.%) annealed at 250 °C, 320 °C, 550 °C, 650 °C, 700 °C, and 800 °C for 720 h were investigated by powder XRD and EDX analyses. For all samples, the main phase had the Ce_20.7Pt_39.9Al_39.4 (at.%) composition. The microstructure of the studied samples showed that the sample annealed at 800 °C was single-phase (Figure 2a), while those annealed at 250 °C, 320 °C, 550 °C, 650 °C, and 700 °C contained an additional unknown phase with a composition close to Ce_23.9Pt_50.7Al_25.4 (at.%).

Microstructures of all samples are shown in Figure S1 in the Supplementary Materials. Microstructure of the Ce_14.2Pt_42.9Al_42.9 (at.%) alloy annealed at 700 °C showed that, in addition, to the main Ce_14.4Pt_42.9Al_42.7 (at.%) phase, the sample contained Pt_50.2Al_49.8 (at.%) and Ce_20.0Pt_40.4Al_39.6 (at.%) as admixtures (Figure 2b).

According to powder XRD patterns, all samples of CePt₂Al₂ including the as-cast one, were single-phase and solely contained an orthorhombic modification of CePt₂Al₂ (Figure 1b, Figure S2a–g). As follows from the XRD pattern of CePt₃Al₃ after annealing, the sample contained PtAl and CePt₂Al₂ admixtures in the amount of 4 mass % and 9 mass %, respectively (Figure 1c).

3.2. Thermal Analysis and Temperature-Dependent XRD

Since two crystallographic modifications were identified for the CePt₂Al₂ compound, tetragonal and orthorhombic, additional studies of the phase transition of CePt₂Al₂ were conducted. DTA (22–1200 °C) was performed for a sample annealed at 550 °C. The heating curve showed a weak endothermic effect at 280(1) °C, which could be attributed to a structural transition from a low-temperature polymorph to a high-temperature one (Figure S3). The endothermal effect at 1100 °C corresponded to the melting point. Attempts to obtain a high-temperature polymorph of CePt₂Al₂ by thermal quenching in cold water failed.

To study the stability of the crystallographic phases of CePt₂Al₂ and their structural transformation, in situ temperature-dependent synchrotron X-ray diffraction measurements were performed. Figure 3a,b clearly demonstrates the changes in X-ray patterns that occurred between 250 and 300 °C.

XRD patterns observed within the range of 25–250 °C corresponded to the low-temperature orthorhombic modification, lt-CePt₂Al₂. However, a change was detected at 300 and 350 °C that indicates a transition to a tetragonal modification, ht-CePt₂Al₂. The second series of in situ X-ray experiments with the same sample within a temperature range of 220–320 °C with 10° incremental increases in temperature (Figure S4a,b) demonstrated a transition at 280 °C. These data strongly support the results observed with DTA measurements and together clearly demonstrate the temperature at which structural phase transition occurs, providing proof of its reversible nature. Further analyses of powder XRD patterns collected at 300 and 350 °C yielded the crystal structure of ht-CePt₂Al₂ (Figure 1a).

3.3. CePt₂Al₂ Crystal Structures

High-temperature modification of CePt₂Al₂ is a new representative of tetragonal CaBe₂Ge₂ type (space group P4/nmm, Pearson symbol tP10) with lattice parameters: a = 4.3637(9) Å, c = 10.0925(14) Å, and Z = 2 (Figure 1a). The lt-CePt₂Al₂ compound crystallizes with its own structure type (space group Cmme, Z = 4): a = 5.84138(2) Å, b = 6.39099(3) Å, c = 10.11611(5) Å (Figure 1b).

Though the general motif of the atomic arrangement in two polymorphs of CePt₂Al₂ seems very similar, some structure peculiarities can be pointed out.

ht-CePt₂Al₂. Following the CaBe₂Ge₂ type, ht-CePt₂Al₂ is constructed from two types of corrugated [Pt₂Al₂] layers perpendicular to [001], with cerium atoms situated between them (Figure 4a,b). In the Pt-based layer, interatomic distances Pt1–Al2 are equal to 2.420(7) Å, and in the Al-based layer, interatomic distances Pt2–Al1 are equal to 2.5544(15) Å, indicating significant chemical bonding in the layers. Neighboring [Pt₂Al₂] layers of two types are connected by Pt2–Al2 contacts that are slightly longer (2.672(17) Å).

lt-CePt₂Al₂. The orthorhombic modification lt-CePt₂Al₂ is a distorted variant of the high-temperature modification (Figure 4c,d). Symmetry reduction from tetragonal to orthorhombic involves differentiation of the lattice parameters a_lt and b_lt, which comprise diagonals a + b of the tetragonal unit cell of ht-CePt₂Al₂. Parameters a_lt and b_lt are related to those of the high-temperature polymorph as a_lt

\approx \sqrt{2}

a_ht and b_lt

\approx \sqrt{2}

a_ht with a_lt < b_lt. Parameter c remains relatively unchanged. The volume of the lt-CePt₂Al₂ unit cell is twice that of the ht-CePt₂Al₂ unit cell. The interatomic distances are similar to those observed in ht-CePt₂Al₂: Pt1–Al2 of 2.416(3) Å and Pt2–Al1 of 2.5313(6) Å within the layers, and Pt2–Al2 of 2.673(3) Å between the layers.

The Ce-centered polyhedra in both polymorphs can be described as hexagonal prisms of eight Pt and eight Al atoms with four additional atoms capping the side faces of the prisms. The range of Ce–Al and Ce–Pt bonding contacts are bigger in the structure of lt-CePt₂Al₂ compared to those in the ht-CePt₂Al₂ at 3.1727(7)–3.503(3) Å and 3.302(3)–3.402(7) Å, respectively. The platinum centered polyhedra can be regarded as a slightly distorted cuboctahedra (Pt1) and tetragonal antiprisms with one additional atom (Pt2). Aluminum atoms are located inside the distorted cuboctahedra (Al1) and mono-caped tetragonal antiprisms (Al2) (Table 3).

3.4. CePt₂Al₂ Phase Transition

The observed phase transition can be attributed to a second-order transition. The space group of lt-CePt₂Al₂ (Cmme) is a subgroup of ht-CePt₂Al₂ (P4/nmm). The main relationship in Bärnighausen formalism [32,33] is presented in Figure 5.

The phase transition is of a displacive nature. Both modifications have a common structural motif and the same local atomic environment. On heating lt-CePt₂Al₂, Pt and Al atoms slightly shift in the directions indicated by the arrows in Figure 6, which leads to the equalization of the Pt1–Pt1, Al1–Al1, Ce–Pt2, and Ce–Al2 interatomic distances and of parameters a and b, and consequently, to transition from an orthorhombic to a tetragonal unit cell (Figure 7a, Table 3). There is no appreciable volume reduction in phase transformation. The formula unit volume increases continuously when heating from 25 °C to 350 °C with a negligible jump at the transition temperature (Figure 7b).

A similar structural phase transition from the orthorhombic modification (Cmme) to the tetragonal modification (P4/nmm) for compounds with palladium—LaPd₂Al₂ and CePd₂Al₂—occurs at 91.5 (5) K and 13.5 (1) K, respectively [22]. Based on a comparison of cell dimensions, one can extrapolate that lt-CePt₂Al₂ is iso-structural with lt-CePd₂Al₂. The crystal structure of the latter compound was not determined. The difference between a_orth and b_orth for lt-CePt₂Al₂ is equal to 0.55 Å, which is appreciably larger when compared to those for lt-LaPd₂Al₂ and lt-CePd₂Al₂ (0.12 Å and 0.14 Å, respectively).

3.5. CePt₃Al₃ Crystal Structure

The structure of CePt₃Al₃ reflects a distorted variant of the iso-stoichiometric CePd₃Al₃ compound [23] and crystallizes with its own type in the orthorhombic cell with dimensions a = 6.36548(6) Å, b = 5.78301(6) Å, c = 13.36245(19) Å, sp. gr. Cmme, Z = 4 (Figure 8a,b). Cell metrics of the CePt₃Al₃ and CePd₃Al₃ compounds correlate as follows: a(CePt₃Al₃)

\approx

\sqrt{2}

a(CePd₃Al₃), b(CePt₃Al₃)

\approx

\sqrt{2}

a(CePd₃Al₃), c(CePt₃Al₃)

\approx

c(CePd₃Al₃), similar to the relationship between the metrics of lt-CePt₂Al₂ and ht-CePt₂Al₂. The group–subgroup relationship in Bärnighausen formalism [32,33] for the structures CePt₃Al₃ and CePd₃Al₃ is presented in Figure 8c. DTA of the CePt₃Al₃ sample did not demonstrate a thermal effect that indicated a possible phase transition.

Similar to lt-CePt₂Al₂, CePt₃Al₃ contains two types of two-dimensional [Pt₂Al₂] layers separated by Ce atoms. If the Al-based layer wholly complies with that of lt-CePt₂Al₂, two Pt-based layers are condensed to form a double layer in which capping Al atoms form the distorted squares of a planar network between two of those of Pt. The shortest Pt–Al interlayer distance Pt2–Al2 of 2.377(11) Å is significantly smaller than that of lt-CePt₂Al₂ and ht-CePt₂Al₂ (2.673 Å) and other Pt–Al contacts of CePt₃Al₃ of 2.5185(8)–2.640(7) Å (Table 3). A similarly short Pt–Al contact of 2.418(6) Å occurs in the Ce₃Pt₄Al₆ structure [34].

In CePt₃Al₃, coordination polyhedra of Ce, Pt, and Al atoms largely resemble those observed in lt-CePt₂Al₂ and ht-CePt₂Al₂. In the environment of the Al2 atom, an additional Al3 neighbor of the double layer results in the formation of a double-capped tetragonal antiprism around the Al2 atom. The Al3 atom is surrounded by eight platinum atoms with Pt–Al separations ranging within 2.611(7)–2.640(7) Å in the form of a distorted CsCl-like cube. With the next-nearest five neighbors at distances up to 3.1831(3) Å away, a polyhedron derived from the cuboctahedron is formed.

3.6. New Homologous Series

The structures of lt-CePt₂Al₂ and CePt₃Al₃ can be presented as Ce-centered polyhedra, sharing common edges in the c-direction and common hexagonal faces perpendicular to the c-axis (Figure 8d). Alternating along the c-axis, similar adjacent layers are inverted and shifted relative to each other. In the CePt₃Al₃ structure, the double layer of Ce-polyhedra alternate with the [PtAl] layer of CsCl-like distorted cubes (Figure 8d). Ternary compounds of lt-CePt₂Al₂ and CePt₃Al₃ comprise a new homologous series built of structural units of lt-CePt₂Al₂ and CsCl-type: CePt_nAl_n (n = 2, 3). Due to the addition of one [PtAl] layer with a thickness of 3.138 Å to the lt-CePt₂Al₂ structure, the c parameter of the unit cell expands from 10.11611(5) Å in lt-CePt₂Al₂ to 13.36245(19) Å in CePt₃Al₃. Homologous series of iso-stoichiometric palladium compounds [23] contains one more member (n = 4), which is composed of alternating double Ce-polyhedra and double [PdAl] layers. An iso-stoichiometric compound with platinum was not observed.

3.7. Crystal Structures of Cerium Platinum Aluminides with High Al Content

The crystal structures analyzed consist of three-dimensional networks of Pt and Al forming Ce-centered hexagonal prisms of alternating Pt and Al atoms at the vertices, which were also observed in the structures of cerium platinum aluminides with high Al content: CePtAl₃ [35], CePt₃Al₅ [36], Ce₄Pt₉Al₁₃ [37], and Ce₂Pt₉Al₁₆ [38] (Figure 9). In these structures, one of the unit cell parameters is about 4.2 Å, which corresponds to the height of the Ce-hexagonal prism. In the structures of ht-CePt₂Al₂, lt-CePt₂Al₂, and CePt₃Al₃ as well as in CePtAl₃, there are two-dimensional layers of condensed Ce-centered hexagonal prisms, in contrast to the infinite isolated single channels of hexagonal prisms in CePt₃Al₅ and Ce₂Pt₉Al₁₆, and combinations of single and condensed triple channels of hexagonal prisms in the Ce₄Pt₉Al₁₃ compound (Figure 9).

4. Conclusions

Cerium platinum aluminides were synthesized. DTA and in situ temperature-dependent synchrotron X-ray diffraction measurements showed a reversible phase transition from a low-temperature orthorhombic CePt₂Al₂ of its own type to a high-temperature tetragonal CePt₂Al₂ of a CaBe₂Ge₂ type when heated to a temperature above 280 °C. The phase transition is of a displacive nature and associated with slight distortions of the [Pt₂Al₂] layers. Orthorhombic compounds CePt₂Al₂ and CePt₃Al₃ present a new homologous series CePt_nAl_n (n = 2, 3) formed from fragments of lt-CePt₂Al₂ and CsCl types.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4352/10/6/465/s1, Figure S1: The microstructure of the Ce_20.0Pt_40.0Al_40.0 (at.%) samples annealed at 250 °C, 320 °C, 550 °C, 650 °C, and 700 °C; Figure S2(a–h): XRD patterns of Ce_20.0Pt_40.0Al_40.0 (at.%) samples; Figure S3: DTA heating thermogram of the CePt₂Al₂ sample; Figure S4: (a,b) Structural transition of low-temperature orthorhombic CePt₂Al₂ to a high-temperature tetragonal modification. XRD patterns at 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320 °C (a); a projection of XRD patterns (b).

Author Contributions

Conceptualization, E.M. and Y.M.; Methodology, E.M. and A.T.; Formal analysis, Y.M. and Z.K.; Investigation, E.M., Y.M., Z.K., and A.T.; Resources, E.M.; Writing—original draft preparation, E.M.; Writing—review and editing, A.T.; Visualization, Y.M.; Supervision, E.M. and S.D.; Project administration, E.M.; Funding acquisition, E.M. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Russian Foundation for Basic Researches (Grant No. 19-03-00135a).

Acknowledgments

The experimental data for the X-ray structure analysis were obtained using equipment at the Shared Physical Characterization Facilities Center, Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences. The authors are indebted to ESRF-Grenoble for providing access to the ID22 Station (experiment MA-3313).

Conflicts of Interest

The authors have no conflicts of interest to declare. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in manuscript composition, or in the decision to publish the results.

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Figure 1. Observed (red dots), calculated (black solid line), and difference (bottom blue line) powder X-ray diffraction (XRD) patterns for tetragonal CePt₂Al₂ (a), orthorhombic CePt₂Al₂ (b), and CePt₃Al₃ (c).

Figure 2. Microstructure of the Ce_20.0Pt_40.0Al_40.0 (at.%) alloy annealed at 800 °C (a) and of the Ce_14.2Pt_42.9Al_42.9 (at.%) alloy annealed at 700 °C (b) obtained using scanning electron microscopy (SEM).

Figure 3. Structural transition of the low-temperature orthorhombic CePt₂Al₂ to the high-temperature tetragonal modification. (a) XRD patterns measured at 25, 100, 150, 200, 250, 300, and 350 °C; (b) a projection of XRD patterns.

Figure 4. Crystal structures of two CePt₂Al₂ modifications: projection of ht-CePt₂Al₂ along the b-axis (a), projection of lt-CePt₂Al₂ in [110] (c), projections along the c-axis of the Pt-based layer of ht-CePt₂Al₂ (b), and of lt-CePt₂Al₂ (d). The Al2 atoms positioned above two-dimensional Pt-based basal layer are pink and those positioned below are rose. Unit cells are outlined in red.

Figure 5. Group–subgroup scheme in the Bärnighausen formalism for the structures of ht-CePt₂Al₂ and lt-CePt₂Al₂.

Figure 6. Projection of the crystal structure of lt-CePt₂Al₂ onto the (001) plane. The arrows indicate the direction of atomic displacements that lead to the transition to the tetragonal ht-CePt₂Al₂ modification. The orthorhombic unit cell is outlined in red, and the tetragonal cell with a black dashed line.

Figure 7. Temperature-dependent evolution of the unit cell dimensions in CePt₂Al₂ (a) and of the scaled unit cell volume V/Z (Z is the formula unit per unit cell) (b). The error bars are smaller than the size of the plotted symbols and range from 0.0002 to 0.003 Å for parameters and from 0.3 to 0.6 Å³ for scaled unit cell volumes.

Figure 8. Crystal structure of CePt₃Al₃, with projection in [110] (a) and projection along the c-axis (b). Unit cells are outlined in red. Group–subgroup relations in the structures of CePt₃Al₃ and CePd₃Al₃ (c). Homologous series of CePt_nAl_n (n = 2, 3) (d).

Figure 9. Projections of networks of crystal structures of ht-CePt₂Al₂, CePtAl₃, Ce₄Pt₉Al₁₃, CePt₃Al₅, and Ce₂Pt₉Al₁₆ in a direction along the smallest unit cell parameter and along [110] for lt-CePt₂Al₂ and CePt₃Al₃ structures. Ce, Pt, and Al atoms are drawn as green, black, and rose spheres, respectively. Single and triple Ce-atom channels and 2D Ce-atoms layers are highlighted in yellow.

Table 1. Crystal data and structural refinement for the ht-CePt₂Al₂, lt-CePt₂Al₂, and CePt₃Al₃ compounds.

Empirical Formula	ht-CePt₂Al₂	lt-CePt₂Al₂	CePt₃Al₃ *
Molar mass, g/mol	584.26	584.26	806.33
Structure type, Pearson symbol	CaBe₂Ge₂, tP10	CePt₂Al₂, oC20	CePt₃Al₃, oC28
Space group, Z	P4/nmm (129), 2	Cmme (67), 4	Cmme (67), 4
Unit cell dimensions
a, Å	4.3637(9)	5.84138(2)	6.36548(6)
b, Å	4.3637(9)	6.39099 (3),	5.78301(6)
c, Å	10.0925(14)	10.11611(5)	13.36245(19)
V, Å³ Calculated density, g/cm³	192.18(6) 10.097	377.657 (3)10.276	491.894(10) 10.888
T, K	623(1)	295(1)	298(2)
Radiation, λ, Å	synchrotron, 0.399962(13)	CuKα₁, 1.54056	CuKα₁, 1.54056
2θ range, step°	2–22.912, 0.002	5–95.19, 0.01	3–93.09, 0.01
Total no. reflections	79	115	162
Refined parameters no.	35	12	29
Rietveld reliability factors
R_p	0.060	0.024	0.037
R_wp	0.069	0.035	0.050
R_exp	0.060	0.016	0.020
χ²	1.334	5.54	5.99

* All indicators—R-factors, no. of parameters, etc. are given for the three-phases refinement.

Table 2. Atomic coordinates and isotropic displacement parameters in the crystal structures of ht-CePt₂Al₂, lt-CePt₂Al₂, and CePt₃Al₃.

Atom	Multiplicity, Wyckoff Letter, Site Symmetry	x/a	y/b	z/c	Uiso., Å²
ht-CePt₂Al₂
Ce1	2c(4mm)	1/4	1/4	0.7456(3)	0.0211(9)
Pt1	2b(-4m2)	3/4	1/4	1/2	0.0211(9)
Pt2	2c(4mm)	1/4	1/4	0.1316(3)	0.0211(9)
Al1	2a(-4m2)	3/4	1/4	0	0.0211(9)
Al2	2c(4mm)	1/4	1/4	0.3963(17)	0.0211(9)
lt-CePt₂Al₂
Ce1	4g(mm2)	0	1/4	0.24780(15)	0.0117(4)
Pt1	4a(222)	1/4	1/2	0	0.0224(4)
Pt2	4g(mm2)	0	1/4	0.62972(11)	0.0146(3)
Al1	4b(222)	1/4	1/2	1/2	0.019(2)
Al2	4g(mm2)	0	1/4	0.8940(7)	0.013(2)
CePt₃Al₃
Ce1	4g(mm2)	0	1/4	0.30545(16)	0.0097(7)
Pt1	8l(…2)	1/4	1/2	0.11280(8)	0.0046(4)
Pt2	4g(mm2)	0	1/4	0.59815(12)	0.0064(5)
Al1	4b(222)	1/4	1/2	1/2	0.001(3)
Al2	4g(mm2)	0	1/4	0.7760(8)	0.039(5)
Al3	4g(mm2)	0	1/4	−0.0019(9)	0.057(5)

Table 3. Selected interatomic distances (d) in ht-CePt₂Al₂, lt-CePt₂Al₂, and CePt₃Al₃ structures.

ht-CePt₂Al₂			lt-CePt₂Al₂			CePt₃Al₃
Atom 1	Atom 2	d, Å	Atom 1	Atom 2	d, Å	Atom 1	Atom 2	d, Å
Ce1	4Pt1	3.302(3)	Ce1	4Pt1	3.3120(11)	Ce1	4Pt1	3.3540(18)
	4Pt2	3.3251(18)		2Pt2	3.1727(7)		2Pt2	3.1655(11)
	4Al1	3.369(3)		2Pt2	3.4273(7)		2Pt2	3.4335(10)
	4Al2	3.402(7)		4Al1	3.3458(12)		4Al1	3.3736(16)
				2Al2	3.254(3)		2Al2	3.090(4)
				2Al2	3.503(3)		2Al2	3.364(4)
Pt1	4Al2	2.420(7)	Pt1	4Al2	2.416(3)	Pt1	2Al3	2.611(7)
	4Pt1	3.0856(6)		2Pt1	2.9207(10)		2Al2	2.614(6)
	4Ce1	3.302(3)		2Pt1	3.1955(15)		2Al3	2.640(7)
				4Ce	3.3120(11)		2Pt1	2.8915(13)
							Pt1	3.0146(15)
							2Pt1	3.1827(13)
							2Ce1	3.3540(18)
Pt2	4Al1	2.5544(15)	Pt2	4Al1	2.5313(6)	Pt2	Al2	2.377(11)
	Al2	2.672(17)		Al2	2.673(7)		4Al1	2.5185(8)
	4Ce1	3.3251(18)		2Ce1	3.1727(7)		2Ce1	3.1655(11)
				2Ce1	3.4273(7)		2Ce1	3.4335(10)
Al1	4Pt2	2.5544(15)	Al1	4Pt2	2.5313(6)	Al1	4Pt2	2.5185(8)
	4Al1	3.0856(6)		2Al1	2.9207(10)		2Al1	2.8915(13)
	4Ce1	3.369(3)		2Al1	3.1955(15)		2Al1	3.1827(13)
				4Ce1	3.3458(12)		4Ce	3.3736(16)
Al2	4Pt1	2.420(7)	Al2	4Pt1	2.416(3)	Al2	Pt2	2.377(11)
	Pt2	2.672(17)		Pt2	2.673(7)		4Pt1	2.614(6)
	4Ce1	3.402(7)		2Ce1	3.254(3)		Al3	2.968(16)
				2Ce1	3.503(3)		2Ce1	3.090(4)
							2Ce1	3.364(4)
						Al3	4Pt1	2.611(7)
							4Pt1	2.640(7)
							2Al3	2.8920(3)
							Al2	2.968(16)
							2Al3	3.1831(3)

Further details regarding the investigation of the crystal structures may be obtained from CCDC/FIZ: CSD-1988138 (tetragonal CePt₂Al₂), CSD-1988139 (orthorhombic CePt₂Al₂), and CSD-1988140 (CePt₃Al₃).

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Murashova, E.; Morozova, Y.; Dunaev, S.; Kurenbaeva, Z.; Tursina, A. Ternary Aluminides of a New Homologous Series—CePt₂Al₂ and CePt₃Al₃: Crystal Structures and Thermal Properties. Crystals 2020, 10, 465. https://doi.org/10.3390/cryst10060465

AMA Style

Murashova E, Morozova Y, Dunaev S, Kurenbaeva Z, Tursina A. Ternary Aluminides of a New Homologous Series—CePt₂Al₂ and CePt₃Al₃: Crystal Structures and Thermal Properties. Crystals. 2020; 10(6):465. https://doi.org/10.3390/cryst10060465

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Murashova, Elena, Yuliya Morozova, Sergey Dunaev, Zhanafiya Kurenbaeva, and Anna Tursina. 2020. "Ternary Aluminides of a New Homologous Series—CePt₂Al₂ and CePt₃Al₃: Crystal Structures and Thermal Properties" Crystals 10, no. 6: 465. https://doi.org/10.3390/cryst10060465

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Ternary Aluminides of a New Homologous Series—CePt₂Al₂ and CePt₃Al₃: Crystal Structures and Thermal Properties

Abstract

1. Introduction