The high-pressure phase of CePtAl

2.1 High-pressure investigations

Attempts to obtain a high-pressure/high-temperature phase transition of CePtAl have been conducted at 4.5 and 10.5 GPa (see Section 3). Although phase pure samples of HP-CePtAl in the MgZn₂-type structure could be obtained under the latter conditions, the first conditions resulted in unreacted NP-CePtAl (TiNiSi type) along with a yet unidentified phase. The indexing of the second phase reflections resulted in unit cell dimensions, which could not be indexed by binary or ternary structure types listed in the Pearson database [6]. Energy dispersive X-ray spectroscopy (EDX) analysis, however, confirmed that the chemical composition was still equiatomic CePtAl and no other compositions arise on the high-pressure/high-temperature treatment (Fig. 1).

Fig. 1:

Scanning electron microscopic image of HP-CePtAl synthesized at 4.5 GPa: (left) secondary electron image; (right) back scattered electron image.

2.2 Crystal structure

NP-CePtAl crystallizes in the orthorhombic TiNiSi-type structure with space group Pnma and lattice parameters of a=719.98 pm, b=448.17 pm, and c=779.38 pm [23]. High-pressure/high-temperature treatment resulted in the formation of HP-CePtAl, which crystallizes in the hexagonal Laves phase (MgZn₂ type, P6₃/mmc) with lattice parameters of a=552.7(1) pm and c=898.8(2) pm. Similar transitions from the orthorhombic TiNiSi to the hexagonal MgZn₂-type structure have been observed for HP-EuPdSn [22], HP-EuPd_0.72In_1.28 [24], and HP-EuPt_0.56In_1.44 [24]. The NP-AEZn₂ (AE=Ca–Ba, all KHg₂ type, Imma) compounds were also transformed to MgZn₂-type HP-AEZn₂ [25]. In the crystal structure of the prototype Mg occupies one position (4f) while for the Zn atoms two positions (2a and 6h) exist. Ce is detected on the Mg site in HP-CePtAl, whereas Pt and Al occupy the Zn sites. Because of the retained composition of 1-1-1, platinum/aluminum mixing on the Zn sites has to take place. The unit cell of CePt_0.98(1)Al_1.02(1) is depicted in Fig. 2, emphasizing the network of condensed [Pt_0.49(1)Al_0.51(1)] tetrahedra. These tetrahedra are alternately condensed via common triangular faces and corners in the c direction. The basal triangular faces condense via all corners to Kagomé networks in the ab plane. The distances between the platinum and the aluminum atoms range from 257 (within the Kagomé net) to 282 pm (connecting atoms), which is in line or slightly longer than the sum of the covalent radii for Pt+Al of 254 (129+125) pm [26]. The cerium atoms are located in voids formed by the [Pt_0.49(1)Al_0.51(1)] network and are surrounded by 16 atoms in the shape of a Frank–Kasper polyhedron (Ce@(Pt/Al)₁₂Ce₄) [27], [28]. The interatomic Ce–Ce distances range from 333 to 352 pm, which is significantly shorter compared to 356 and 377 pm in NP-CePtAl and close to the sum of the covalent radii of 330 pm [26]. Taking larger interatomic distances into account, the two Pt/Al sites are surrounded by 12 atoms in the shape of icosahedra (Pt/Al@(Pt/Al)₆Ce₆). Details of the structure refinement are listed in Tables 1–3.

Fig. 2:

Crystal structure of HP-CePtAl in the MgZn₂-type structure. (Top) Extended unit cell depicting the Kagomé nets in z=1/4 and 3/4 along with the trigonal bipyramidal coordination environments. (Bottom) View along [001] onto the unit cell. Ce atoms are depicted in blue, Pt atoms in black, and Al atoms in open white circles. The mixed occupation ratios are indicated by the fragmented circles.

The comparably low quality of the single crystals can be ascribed to the low crystallinity of the sample, as seen by powder X-ray diffraction. The broadening of the base of the reflections and a significant contribution to the background, most likely by amorphous parts of the sample, are visible. Therefore, only multidomain crystals could be investigated; the best measurement is presented here.

2.3 Magnetic properties

HP-CePtAl was investigated by magnetic susceptibility measurements at 10 kOe (1 kOe=7.96×10⁴ A m⁻¹). The χ and the χ⁻¹ data are shown in Fig. 3, top panel. A fit of the χ⁻¹ data in the region higher than 150 K using the Curie–Weiss law revealed an effective magnetic moment of μ_eff=2.54(1) μ_B per Ce atom and a Weiss constant of θ_p=−151(5) K. The effective magnetic moment matches the theoretical value for a free Ce³⁺ ion. The large negative value of the Weiss constant is due to crystal field splitting, which is also responsible for the pronounced curvature below 100 K. Crystal field splitting occurs rather frequently in intermetallic cerium compounds because of the proximity of the ground state energies. Prominent examples exhibiting strong crystal field splitting are, e.g. CeAgAl [29], CeRhGe [30], or CePtGe [30]. In the latter two compounds, the negative Weiss constant was also attributed to the presence of the Kondo effect, backed up by resistivity measurements. Because of the small sample size, we were not able to conduct these experiments. To obtain more information about possible ordering phenomena, a low-field measurement was performed in a zero-field- and field-cooled mode (ZFC/FC), which is shown in the bottom panel of Fig. 3. No bifurcation between the ZFC and the FC curves and no magnetic ordering were observed down to 2.5 K. The inset in Fig. 3 (bottom panel) displays the magnetization isotherms of HP-CePtAl measured at 3, 10, and 50 K. The 50 K isotherm displays a linear field dependency of the magnetization as expected for a paramagnetic material, whereas the 3 and 10 K isotherms exhibit slight curvatures. At higher fields, no saturation effects can be observed. The saturation magnetization at 3 K and 80 kOe is μ_sat=0.19(2) μ_B per Ce atom, which is drastically below the expected saturation magnetization of 2.14 μ_B per Ce according to g_J×J, however, in line with that of several other intermetallic cerium compounds, e.g. Ce₁₆Au₃Al₁₀ [31].

Fig. 3:

Magnetic properties of HP-CePtAl. (Top) Temperature dependence of the magnetic susceptibility χ and its reciprocal χ⁻¹ measured with a magnetic field strength of 10 kOe. (Bottom) Magnetic susceptibility in zero-field- (ZFC) and field-cooled (FC) mode at 100 Oe. (Inset) Magnetization isotherms recorded at 3, 10, and 50 K.

3.1 Synthesis

CePtAl was synthesized directly from the elements using pieces of cerium metal (Smart Elements), platinum sheets (Allgemeine Gold- und Silberscheideanstalt, Pforzheim), and aluminum turnings (Koch Chemicals), all with stated purities higher than 99.5%. The cerium ingots were cut into smaller pieces and first arc melted [32] to a button under argon (approximately 800 mbar). The argon was purified with titanium sponge (873 K), silica gel, and molecular sieves. The cerium button was subsequently mixed with pieces of the platinum sheet and the aluminum turnings in the ideal 1:1:1 atomic ratio and reacted in the same arc-melting furnace. The product buttons were remelted several times to ensure homogeneity. The purity of the precursor compound was checked by powder X-ray diffraction (vide infra) to ensure that only X-ray pure samples were used for the subsequent high-pressure experiments.

3.2 High-pressure/high-temperature syntheses

The high-pressure/high-temperature treatments were conducted via a multianvil press, equipped with a Walker-type module. Details about the technique and the construction of the different assemblies can be found in numerous references [33], [34], [35], [36]. Carefully milled powders of NP-CePtAl were loaded into 14/8 assembly crucibles made of hexagonal boron nitride and compressed within 67 and 250 min to pressures of 4.5 and 10.5 GPa, respectively. Subsequent heating to 1620 K within 15 min was followed by a period of 10 min at a constant temperature. Afterward, the samples were gently cooled down to 970 K for the next 120 min (4.5 GPa experiment) or 90 min (10.5 GPa experiment). This annealing process under pressure at elevated temperatures is important to enhance the crystallinity of the samples and was followed by quenching the samples to room temperature. After decompression, the samples were carefully separated from the surrounding assembly parts by mechanical fragmentation. The polycrystalline samples are silvery with metallic lustre. Powdered samples of both products are dark gray and stable in air.

3.3 EDX analyses

The HP-CePtAl sample was studied by electron microscopy with attached back scattering devices because high-pressure decomposition was observed in the HP-EuTIn (T=Ni, Pd) system [24]. A piece of the high-pressure sample that adhered to the boron nitride crucible was studied. The surface was left unetched for the electron-microscopic characterization. An electron microscopic image and a 200×300-μm cutout of the unpolished surface taken in backscattering mode are presented in Fig. 4. EDX data at various points of the surface showed a homogeneous composition of Ce/Pt/Al, 31(1):34(1):35(1) at%.

Fig. 4:

Scanning electron microscopic image of HP-CePtAl synthesized at 10.5 GPa. (Left) Boron nitride (BN) crucible with adhered HP-CePtAl. (Right) Enlarged BSE image of HP-CePtAl.

The single crystal measured on the diffractometer was analyzed semiquantitatively using a Zeiss EVO^® MA10 scanning electron microscope with Ce₂O₃, Pt, and Al₂O₃ as standards. No impurity elements heavier than sodium (detection limit of the instrument) were observed. The experimentally determined element ratios Ce/Pt/Al (32:34:34 at%) were within 2 at% of the theoretical values.

3.4 X-ray diffraction

The polycrystalline samples were characterized by Guinier patterns (imaging plate detector, Fujifilm BAS-1800 scanner) with CuKα₁ radiation using α-quartz (a=491.30, c=540.46 pm, Riedel-de-Häen) as an internal standard. The correct indexing of the diffraction lines was ensured through intensity calculations. The lattice parameters were obtained through least squares fits [37]. Small block-shaped crystals of HP-CePtAl were selected from the samples by the mechanical fragmentation of the high-pressure button. The crystals were glued to thin quartz fibers using beeswax and first investigated by Laue photographs on a Buerger camera (white molybdenum radiation, image plate technique; Fujifilm, BAS-1800) to check their quality. Intensity data sets were collected at room temperature by use of a Stoe StadiVari four-circle diffractometer (μ-source; MoKα radiation, λ=71.073 pm; oscillation mode) with an open Eulerian cradle setup equipped with a reverse-biased silicon diode array detector (Dectris Pilatus 100 K; resolution, 487×195 pixel; pixel size, 0.172×0.172 mm² [38]) in oscillation mode. Numerical absorption correction was applied to the data set. Details of the data collections and the structure refinements are listed in Tables 1–3.

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD-432027.

3.5 Magnetic properties

A polycrystalline piece of the HP/HT-treated ingot was packed in Kapton foil and attached to the sample holder rod of a vibrating sample magnetometer unit for measuring the magnetization M(T,H) in a quantum design Physical Property Measurement System. The samples were investigated in the temperature range of 2.5–300 K and with magnetic flux densities up to 80 kOe.