Ternary Aluminides of a New Homologous Series—CePt 2 Al 2 and CePt 3 Al 3 : Crystal Structures and Thermal Properties

: In the process of studying the Ce–Pt–Al system, we identiﬁed CePt 2 Al 2 and CePt 3 Al 3 , two new ternary intermetallic compounds. CePt 2 Al 2 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 (CaBe 2 Ge 2 type, P 4 / nmm , a = 4.3637(9) Å, c = 10.0925(14) Å) at 280(1) ◦ C. CePt 3 Al 3 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 CePt 2 Al 2 -type and CsCl-type.

The structure of ThCr 2 Si 2 is an ordered version of BaAl 4 with more than 1700 ternary intermetallics being known as isotypic. In the ThCr 2 Si 2 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 [Cr 2 Si 2 ] layers are inverted with respect to each other and are separated by Th atoms. The structure remains I-centered like the BaAl 4 prototype.
In the structure of CaBe 2 Ge 2 , 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 [Be 2 Ge 2 ] layers, CaBe 2 Ge 2 has a primitive unit cell. Remarkably, CaBe 2 Ge 2 type intermetallics are more likely to demonstrate superconductivity at high temperatures [11].
Intermetallics with platinum-REPt 2 X 2 most commonly crystallize in the CaBe 2 Ge 2 type. Silicides REPt 2 Si 2 (RE = Y, La−Nd, Sm, Gd-Lu, U, Th) crystallize in a CaBe 2 Ge 2 type and do not undergo

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 CePt 2 Al 2 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 CePt 3 Al 3 was annealed in an evacuated ampoule at 700 • C for 720 h.

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 Crystals 2020, 10, 465 3 of 15 on a cloth with an Al 2 O 3 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.%.

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 • .

Crystal Structure Determination
The crystal structures of the tetragonal and orthorhombic modifications of CePt 2 Al 2 as well as CePt 3 Al 3 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 CePt 2 Al 2 . The preliminary parameters of tetragonal CePt 2 Al 2 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 CaBe 2 Ge 2 type.
The tetragonal unit cell parameters obtained from powder XRD (Table 1) were compatible with the structure types of ThCr 2 Si 2 and CaBe 2 Ge 2 . Analysis of the systematic reflection conditions indicated a primitive unit cell, therefore the CaBe 2 Ge 2 type was chosen as a structural model. For the Rietveld refinement of tetragonal CePt 2 Al 2 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.
Crystals 2020, 10, x FOR PEER REVIEW 3 of 16 sizes and then polished on a cloth with an Al2O3 paste. Finally, the tablet was washed for 5 minutes in an ultrasonic bath filled with ethanol. The uncertainty of measurements for each element did not exceed 0.7 at.%.

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°.

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 CePt 2 Al 2 and CePt 3 Al 3 . Careful examination of the systematic extinctions in the orthorhombic CePt 2 Al 2 and CePt 3 Al 3 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 CePt 2 Al 2 and CePt 3 Al 3 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 CePt 2 Al 2 and for three phases in the case of CePt 3 Al 3 . For the latter, small impurities that had previously been detected by EDX (PtAl binary and orthorhombic CePt 2 Al 2 ) 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 CePt 2 Al 2 , orthorhombic CePt 2 Al 2 , and CePt 3 Al 3 are listed in Table 2, and selected interatomic distances are given in Table 3.

Differential Thermal Analysis
Thermal stability and temperature at which the structural phase transition of CePt 2 Al 2 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. °C, 550 °C, 650 °C, and 700 °C contained an additional unknown phase with a composition close to Ce23.9Pt50.7Al25.4 (at.%). Microstructures of all samples are shown in Figure S1 in the Supplementary Materials. Microstructure of the Ce14.2Pt42.9Al42.9 (at.%) alloy annealed at 700 °C showed that, in addition, to the main Ce14.4Pt42.9Al42.7 (at.%) phase, the sample contained Pt50.2Al49.8 (at.%) and Ce20.0Pt40.4Al39.6 (at.%) as admixtures (Figure 2b).

Sample Characterization
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 (Figures 1b, 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).

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 lowtemperature 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. According to powder XRD patterns, all samples of CePt 2 Al 2 including the as-cast one, were single-phase and solely contained an orthorhombic modification of CePt 2 Al 2 (Figure 1b, Figure S2a-g). As follows from the XRD pattern of CePt 3 Al 3 after annealing, the sample contained PtAl and CePt 2 Al 2 admixtures in the amount of 4 mass % and 9 mass %, respectively (Figure 1c).

Thermal Analysis and Temperature-Dependent XRD
Since two crystallographic modifications were identified for the CePt 2 Al 2 compound, tetragonal and orthorhombic, additional studies of the phase transition of CePt 2 Al 2 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 CePt 2 Al 2 by thermal quenching in cold water failed.
To study the stability of the crystallographic phases of CePt 2 Al 2 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-CePt 2 Al 2 . However, a change was detected at 300 and 350 • C that indicates a transition to a tetragonal modification, ht-CePt 2 Al 2 . 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-CePt 2 Al 2 (Figure 1a). 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).
Though the general motif of the atomic arrangement in two polymorphs of CePt2Al2 seems very similar, some structure peculiarities can be pointed out.
Though the general motif of the atomic arrangement in two polymorphs of CePt 2 Al 2 seems very similar, some structure peculiarities can be pointed out.
lt-CePt 2 Al 2 . The orthorhombic modification lt-CePt 2 Al 2 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-CePt 2 Al 2 . Parameters a lt and b lt are related to those of the high-temperature polymorph as a lt ≈ √ 2 a ht and b lt ≈ √ 2 a ht with a lt < b lt . Parameter c remains relatively unchanged. The volume of the lt-CePt 2 Al 2 unit cell is twice that of the ht-CePt 2 Al 2 unit cell. The interatomic distances are similar to those observed in ht-CePt 2 Al 2 : 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-CePt 2 Al 2 compared to those in the ht-CePt 2 Al 2 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).  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).

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.

CePt 2 Al 2 Phase Transition
The observed phase transition can be attributed to a second-order transition. The space group of lt-CePt 2 Al 2 (Cmme) is a subgroup of ht-CePt 2 Al 2 (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 The phase transition is of a displacive nature. Both modifications have a common structural motif and the same local atomic environment. On heating lt-CePt 2 Al 2 , 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). 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 aorth and borth 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). A similar structural phase transition from the orthorhombic modification (Cmme) to the tetragonal modification (P4/nmm) for compounds with palladium-LaPd 2 Al 2 and CePd 2 Al 2 -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-CePt 2 Al 2 is iso-structural with lt-CePd 2 Al 2 . The crystal structure of the latter compound was not determined. The difference between a orth and b orth for lt-CePt 2 Al 2 is equal to 0.55 Å, which is appreciably larger when compared to those for lt-LaPd 2 Al 2 and lt-CePd 2 Al 2 (0.12 Å and 0.14 Å, respectively).

CePt 3 Al 3 Crystal Structure
The structure of CePt 3 Al 3 reflects a distorted variant of the iso-stoichiometric CePd 3 Al 3 compound [23] and crystallizes with its own type in the orthorhombic cell with dimensions a = 6.36548 (6)  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. Similar to lt-CePt 2 Al 2 , CePt 3 Al 3 contains two types of two-dimensional [Pt 2 Al 2 ] layers separated by Ce atoms. If the Al-based layer wholly complies with that of lt-CePt 2 Al 2 , 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-CePt 2 Al 2 and ht-CePt 2 Al 2 (2.673 Å) and other Pt-Al contacts of CePt 3 Al 3 of 2.5185(8)-2.640(7) Å (Table 3). A similarly short Pt-Al contact of 2.418(6) Å occurs in the Ce 3 Pt 4 Al 6 structure [34].
In CePt 3 Al 3 , coordination polyhedra of Ce, Pt, and Al atoms largely resemble those observed in lt-CePt 2 Al 2 and ht-CePt 2 Al 2 . 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.

New Homologous Series
The structures of lt-CePt 2 Al 2 and CePt 3 Al 3 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 CePt 3 Al 3 structure, the double layer of Ce-polyhedra alternate with the [PtAl] layer of CsCl-like distorted cubes (Figure 8d). Ternary compounds of lt-CePt 2 Al 2 and CePt 3 Al 3 comprise a new homologous series built of structural units of lt-CePt 2 Al 2 and CsCl-type: CePt n Al n (n = 2, 3). Due to the addition of one [PtAl] layer with a thickness of 3.138 Å to the lt-CePt 2 Al 2 structure, the c parameter of the unit cell expands from 10.11611(5) Å in lt-CePt 2 Al 2 to 13.36245(19) Å in CePt 3 Al 3 . 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.

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: CePtAl 3 [35], CePt 3 Al 5 [36], Ce 4 Pt 9 Al 13 [37], and Ce 2 Pt 9 Al 16 [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-CePt 2 Al 2 , lt-CePt 2 Al 2 , and CePt 3 Al 3 as well as in CePtAl 3 , there are two-dimensional layers of condensed Ce-centered hexagonal prisms, in contrast to the infinite isolated single channels of hexagonal prisms in CePt 3 Al 5 and Ce 2 Pt 9 Al 16 , and combinations of single and condensed triple channels of hexagonal prisms in the Ce 4 Pt 9 Al 13 compound ( Figure 9). layers. An isostoichiometric compound with platinum was not observed.

Crystal Structures of Cerium Platinum Aluminides with High Al Content
The crystal structures analyzed consist of three-dimensional networks of Pt and Al forming Cecentered 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).

Conclusions
Cerium platinum aluminides were synthesized. DTA and in situ temperature-dependent synchrotron x-ray diffraction measurements showed a reversible phase transition from a lowtemperature orthorhombic CePt2Al2 of its own type to a high-temperature tetragonal CePt2Al2 of a

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 CePt 2 Al 2 of its own type to a high-temperature tetragonal CePt 2 Al 2 of a CaBe 2 Ge 2 type when heated to a temperature above 280 • C. The phase transition is of a displacive nature and associated with slight distortions of the [Pt 2 Al 2 ] layers. Orthorhombic compounds CePt 2 Al 2 and CePt 3 Al 3 present a new homologous series CePt n Al n (n = 2, 3) formed from fragments of lt-CePt 2 Al 2 and CsCl types.