The equiatomic intermetallics REPtCd (RE= La, Ce, Pr, Nd, Eu) and magnetic properties of CeAuCd

2.1 Synthesis

The RETCd (RE = La–Nd, Eu; T = Pt, Au) samples were synthesized directly from the elements. Starting materials were pieces of the rare earth elements (smart elements), platinum sponge and gold drops (Allgemeine Gold- und Silberscheideanstalt, Pforzheim), and a cadmium rod (Sigma-Aldrich), all with stated purities better than 99.9 %. The moisture-sensitive pieces of the rare earth elements were kept under argon in Schlenk tubes prior to the reactions. The ideal starting compositions 1RE:1T:1Cd always led to the Mo₂B₂Fe type RE₂T₂Cd [7] phases as by-products (most likely small amounts of cadmium distilled to the lids of the niobium ampoules). A small cadmium excess of 5–10 mol% was used to compensate the loss. All samples were arc-welded [18] in small niobium tubes under an argon pressure of about 700 mbar. The argon was purified with titanium sponge (870 K), silica gel, and molecular sieves. The niobium ampoules were sealed in evacuated silica tubes for oxidation protection, heated to 1323 K and kept at that temperature for 3 h. The temperature was then lowered to 773 K at a rate of 5 K h^–1 and held for 4 days, followed by quenching. At the end of the annealing sequence all samples could easily be separated from the niobium tubes by mechanical fragmentation. The RETCd samples are slightly sensitive to moisture. They were kept in Schlenk tubes under argon. The powdered samples are dark gray, and single crystals exhibit metallic luster.

2.2 X-ray diffraction

The powdered polycrystalline REPtCd and CeAuCd samples were characterized by X-ray diffraction (Guinier technique: Fujifilm BAS-1800 imaging plate detector, CuK_α1 radiation) with α-quartz (a = 491.30, c = 540.46 pm) as an internal standard. The lattice parameters (Table 1) were obtained from least-squares refinements. Intensity calculations [19] ensured correct indexing.

Irregularly shaped crystals of CePtCd and EuPtCd were selected from the annealed samples by careful mechanical fragmentation. They were glued to quartz fibers using beeswax and investigated by Laue photographs on a Buerger camera (white molybdenum radiation, image plate technique, Fujifilm, BAS-1800) in order to check crystal quality and suitability for intensity data collection. Single-crystal diffraction intensities were collected at room temperature on a four-circle diffractometer (CAD4) with graphite-monochromatized MoK_α radiation (λ = 0.71073 pm) and a scintillation counter with pulse height discrimination. The scans were performed in the ω/2θ mode. Empirical absorption corrections were applied on the basis of ψ-scan data. Details of the data collections and the structure refinements are listed in Table 2.

2.3 EDX data

The CePtCd and EuPtCd crystals studied on the diffractometer were analyzed by semiquantitative energy-dispersive X-ray (EDX) analysis using a Zeiss EVO MA10 scanning electron microscope with CeO₂, EuF₃, platinum, and cadmium as standards. No impurity elements (especially from the niobium crucibles) heavier than sodium (detection limit of the instrument) were observed.

2.4 Structure refinements

Isotypic behavior of CePtCd with GdPdCd [20] and of EuPtCd with EuPdCd [21] was already evident from the powder diffraction data. The atomic parameters of the two palladium compounds were taken as starting values, and both structures were refined using Shelxl-97 [22] (full-matrix least-squares on F²) with anisotropic atomic displacement parameters for all atoms. As a check for possible mixed occupied sites, all occupancy parameters were refined in separate series of least-squares cycles. Since all sites were fully occupied within two standard deviations, the ideal occupancies were assumed again in the final cycles. Refinement of the correct absolute structure of CePtCd was ensured through calculation of the Flack parameter [23, 24]. The final difference Fourier syntheses revealed no significant residual peaks (Table 2). The positional parameters and interatomic distances are listed in Tables 3 and 4. Further information on the structure refinements is available.^[1]

2.5 Magnetic measurements

Temperature-dependent magnetic susceptibility measurements of CeAuCd were carried out on a Quantum Design Physical Property Measurement System using the VSM option. For the measurement the powdered sample (31.510 mg) was packed in a polypropylene capsule and attached to the sample holder rod. The measurement was performed in the temperature range of 3–305 K with magnetic flux densities up to 80 kOe (1 kOe = 7.96 × 10⁴ A m^–1).

3.1 Crystal chemistry

Equiatomic cadmium compounds REPtCd could be obtained with lanthanum, cerium, praseodymium, neodymium, and europium as the rare earth components. LaPtCd, CePtCd, PrPtCd, and NdPtCd crystallize with the hexagonal ZrNiAl type structure [10–12]. The lattice parameters decrease from the lanthanum to the neodymium compound (Fig. 1), a consequence of the lanthanide contraction. Similar behavior occurs for the series RETCd with other transition metals [2]. EuPtCd shows a different structure type. It crystallizes with the orthorhombic TiNiSi type [9] with four formula units per cell. The volume per formula unit is close to that of LaPtCd, indicating divalent europium, similar to the series of REPdCd compounds [21].

Fig. 1

Course of the lattice parameters in the series of hexagonal REPtCd compounds.

Before we discuss the crystal chemistry of the REPtCd compounds in detail, we draw back to the synthesis conditions. The high melting point and the lower reactivity of platinum hamper the synthesis of phase pure samples. In many cases, one observes distillation of cadmium to the lids of the metal ampoules, and this loss of cadmium leads to small amounts of the RE₂Pt₂Cd phases as by-products. Samples with a slight excess of cadmium showed an increase of the yield of the equiatomic REPtCd phases, but still small quantities of by-products (elemental cadmium along with some new ternary compositions as evidenced by EDX) were formed. Unfortunately, these cadmium phases had no sufficient purity for property studies. Only CeAuCd was obtained in X-ray pure form.

The shortest interatomic distances in both structures occur for Pt–Cd with ranges of 279–293 pm (CePtCd) and 281–285 pm (EuPtCd). These distances are all longer than the sum of the covalent radii for platinum and cadmium of 270 pm [3], indicating weaker Pt–Cd bonding, similar to Ce₂Pt₂Cd with 306 pm Pt–Cd [7]. The platinum and cadmium atoms build up three-dimensional [PtCd]^δ– polyanionic networks (Fig. 2) which host the rare earth atoms. Each cadmium atom within these networks has slightly distorted tetrahedral platinum coordination (Fig. 3). The tetrahedra in both structures share common corners and edges, but there are differences between both structures concerning the connectivity patterns. The platinum atoms have between three and six cadmium neighbors. The notations Cd@Pt_2/6Pt_2/3 and Cd@Pt_4/4 account for the different connectivities in CePtCd and EuPtCd, respectively.

Fig. 2

Coordination of the cerium and europium atoms in CePtCd and EuPtCd. The platinum and cadmium atoms are drawn as blue and magenta circles, respectively. Relevant interatomic distances and the two crystallographically independent platinum sites in CePtCd are emphasized.

Fig. 3

Cutout of the CePtCd and EuPtCd structures, emphasizing the connectivity pattern of the Cd@Pt_2/6Pt_2/3 and Cd@Pt_4/4 tetrahedra.

These tetrahedra are also formed in the isotypic indium compounds. Exemplarily we discuss the TiNiSi type structures of EuPtCd (a = 741.3, b = 436.4, c = 858.0 pm) and EuPtIn [25] (a = 746.94, b = 447.27, c = 843.46 pm). The differences in the lattice parameters are caused by the difference in size between cadmium and the slightly larger indium atoms as well as the small changes between Cd–Pt and In–Pt hybridization. The In–Pt distances in EuPtIn range from 280 to 282 pm, close to the sum of the covalent radii of 279 pm for platinum and indium [3]. This is consistent with stronger Pt–In bonding as compared to Pt–Cd. In general, substitution of indium by cadmium or magnesium within such polyanionic networks leads to a weakening of the T–X bonding [26, 27]. Such changes in T–X bonding along with an increase or decrease of the valence electron concentration influence the magnetic behavior of the rare earth element. This is discussed for CeAuCd in the next chapter.

3.2 Magnetic properties of CeAuCd

The top panel of Fig. 4 displays the temperature dependence of the magnetic and inverse magnetic susceptibility (χ and χ^–1 data) measured at 10 kOe. A fit of the χ^–1 data in the region of 20–300 K, using the Curie–Weiss law, revealed an effective magnetic moment of μ_eff = 2.56(1) μ_B per Ce atom and a Weiss constant of θ_p = –1.6(1) K. The negative value of the Weiss constant points toward weak antiferromagnetic interactions in the paramagnetic range, and the effective magnetic moment is in very good agreement with the theoretical value of 2.54 μ_B for a free Ce³⁺ ion.

Fig. 4

Magnetic properties of CeAuCd: (top) temperature dependence of the magnetic susceptibility (χ and χ^–1 data) measured at 10 kOe; (inset) magnetic susceptibility in zero-field- and field-cooled mode at 100 Oe; (bottom) magnetization isotherms at 3, 10, 20, and 50 K.

Low-field measurements were performed in a zero field- and field-cooled mode, and the results are shown in the inset of Fig. 4. No bifurcation between the ZFC and the FC curves is visible, and the Néel temperature could be determined to be T_N = 3.7(5) K, similar to isotypic CeAuMg (T_N = 2.0 K) [28, 29] and CeAuIn (T_N = 5.7 K) [30].

The bottom panel in Fig. 4 displays the magnetization isotherms of CeAuMg measured at 3, 10, 20, and 50 K. The isotherms above the ordering temperature display a linear field dependency of the magnetization as expected for a paramagnetic material. At 3 K the magnetization increases significantly steeper in comparison to the remaining ones and exhibits a slightly S-shaped character. The field-induced spin reorientation confirms the antiferromagnetic ground state, and the critical field can be determined to be around 16(5) kOe. At fields above 40 kOe a tendency for saturation can be observed, and the magnetic moment at 3 K and 80 kOe (0.95(1) μ_B per Ce atom) is much lower than the theoretical saturation magnetization of 2.14 μ_B according to g_J × J. Such reduced magnetization values often occur in cerium compounds and can be attributed to crystal field splitting of the J = 5/2 magnetic ground state of Ce³⁺.