The stannides RE₃Au₆Sn₅ (RE = La, Ce, Pr, Nd, Sm) – synthesis, structure, magnetic properties and ¹¹⁹Sn Mössbauer spectroscopy

2.1 Synthesis

Starting materials for the syntheses of the RE₃Au₆Sn₅ phases were pieces of sublimed rare earth elements (smart elements), gold shots (Agosi, 99.9 %) and tin granules (Merck, >99.99 %). The moisture sensitive rare earth pieces were kept under dry argon in Schlenk tubes prior to the reactions. Argon was purified over titanium sponge (900 K), silica gel and molecular sieves.

Samples with lanthanum and cerium were prepared with the ideal elemental mixture of 3RE: 6Au: 5Sn. The elements were reacted in an arc-melting furnace [19] under an argon pressure of ca. 700 mbar and re-melted three times on each side to ensure homogeneity. The product buttons were then placed in small open tantalum crucibles which were sealed in evacuated silica tubes. The ampoules were annealed at 870 K for 3 weeks followed by quenching to room temperature. The praseodymium, neodymium, and samarium containing samples were obtained from batches of the starting compositions 21RE: 44Au: 35Sn (to avoid formation of the REAuSn and/or RE₂Au₅Sn₂ phases as by-products), arc-melted and annealed in the same sequence. The resulting samples are brittle and exhibit silvery luster while ground powders appear light grey. The samples are stable in air over months.

When searching for isotypic compounds with the heavier rare earth elements we observed the new phases RE₂Au₅Sn₂ [20] with an ordered arrangement of the Au₇Ga₂ type [21].

2.2 X-ray diffraction

The polycrystalline RE₃Au₆Sn₅ samples were studied by X-ray powder diffraction using the Guinier technique: imaging plate detector, Fujifilm BAS-1800, CuK_α1 radiation and α-quartz (a = 491.30, c = 540.46 pm) as an internal standard. The orthorhombic lattice parameters (Table 1) were deduced from least-squares refinements of the Guinier powder data. Proper indexing was ensured by comparison of the experimental patterns with calculated ones [22].

Crystal fragments were selected from the crushed samples with cerium, neodymium, and samarium as the rare earth component. The crystals were characterized on a Buerger camera (using white Mo radiation) to check their quality. Intensity data of a Nd₃Au₆Sn₅ and a Sm₃Au_5.59(2)Sn_5.41(2) crystal were collected at room temperature on a Stoe IPDS-II image plate system (graphite monochromatized Mo radiation; λ = 71.073 pm) in oscillation mode. The Ce₃Au₆Sn₅ data set was measured on a Stoe STADIVARI diffractometer equipped with a Mo micro focus source and a Pilatus detection system and scaled subsequently with reference to the Gaussian-shaped profile of the X-ray source. Numerical absorption corrections (along with scaling for the STADIVARI data set) were applied to the data sets. Details about the data collections and the crystallographic parameters are summarized in Table 2.

The three diffractometer data sets showed primitive orthorhombic lattices and the systematic extinctions were compatible with space group Pmmn. Since isotypism with Ce₃Pd₆Sb₅ [11] was already evident from the X-ray powder data, we used the atomic parameters of the antimonide as starting values. The structures were then refined with anisotropic displacement parameters for all atoms with Shelxl-97 (full-matrix least-squares on F_o²) [23, 24]. As a check for the correct site assignments and/or possible mixed occupancies, all occupancy parameters were refined in separate series of least-squares cycles. For Ce₃Au₆Sn₅ and Nd₃Au₆Sn₅ all sites were fully occupied within two standard deviations and the ideal compositions were assumed again in the following cycles. The samarium-based crystal showed too low scattering power for the 2b Au2 site, indicating mixing with tin. This mixed occupancy was refined as a least-squares variable in the following cycles, leading to the composition Sm₃Au_5.59(2)Sn_5.41(2) for the investigated crystal. Final difference Fourier syntheses revealed no residual peaks. The refined atomic positions, displacement parameters, and interatomic distances (exemplarily for Ce₃Au₆Sn₅) are given in Tables 3 and 4.

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the deposition number CSD-428665 (Ce₃Au₆Sn₅), CSD-428666 (Nd₃Au₆Sn₅) and CSD-428667 (Sm₃Au_5.59Sn_5.41).

2.3 EDX data

Semiquantitative EDX analyses of the single crystals studied on the diffractometers were carried out in variable pressure mode with a Zeiss EVO^® MA10 scanning electron microscope with the rare earth trifluorides, gold, and tin as standards. The experimentally observed compositions were close to the ideal ones. No impurity elements were detected.

2.4 Physical property studies

The magnetic and heat capacity measurements were carried out on a Quantum Design Physical Property Measurement System (PPMS) using dc-MS and heat capacity options, respectively. Dc magnetic measurements were performed using the VSM (Vibrating Sample Magnetometer) option. For these measurements approximately 30 mg of the crushed samples were fixed to the sample holder rod by kapton foil. For the heat capacity measurement a piece of Sm₃Au₆Sn₅ (36.225 mg) was fixed to a pre-calibrated heat capacity puck using Apiezon N grease. Magnetic investigations were performed in the temperature range of 2.5–305 K with magnetic flux densities up to 80 kOe (1 kOe = 7.96 × 10⁴ A m⁻¹). Heat capacity measurements were done in the temperature range of 2.1–310 K.

2.5 ¹¹⁹Sn Mössbauer spectroscopy

A Ca^119mSnO₃ source was used for the Mössbauer spectroscopic experiments. The measurements were carried out at ambient temperature, in a nitrogen bath cryostat in case of 78 K measurements and in a helium flow cryostat to reach temperatures around 5 K. The Mössbauer source was kept at room temperature. A palladium foil of 0.05 mm thickness was used to reduce the tin K X-rays concurrently emitted by this source. The samples were enclosed in small PMMA containers at a thickness corresponding to about 10 mg Sn per cm². Fitting of the data was done by using the Normos-90 program package [25].

2.6 Field gradient calculations

Theoretical electric field gradient calculations were conducted using the WIEN2k code, a full-potential all electron method based on the LAPW+LO method [26]. SCF calculations were done with R_mt parameters of 2.38 atomic units for Sn and 2.5 atomic units for the rest. Separation energies between the core and valence states were set to −6 Ry. The plane wave cutoff parameter Rmtmin × Kmax was optimized in steps of 0.5 units within the range of 5.50–7.00. For Ce₃Au₆Sn₅ a value of 6.50 was used and for Nd₃Au₆Sn₅ a value of 7.00. For describing the first Brillouin zone, 3 k-points were used initially, and this was successively increased up to 400 k-points. 16 816 (Nd₃Au₆Sn₅) and 16 930 (Ce₃Au₆Sn₅) plane waves were used to describe the electronic state of the crystal structures. The nuclear quadrupole moment Q= 13.2(1) fm² used to calculate the quadrupolar coupling constant C_Q for the first exited state (I = 3/2) of the ¹¹⁹Sn nuclei was obtained by Barone et al. [27].

3.1 Crystal chemistry

The stannides RE₃Au₆Sn₅ (RE= La, Ce, Pr, Nd, Sm) crystallize with the orthorhombic Ce₃Pd₆Sb₅ type [11]. The present single crystal data fully confirm the X-ray powder data published recently [14–16]. In the series of antimonides RE₃Pd₆Sb₅ also the representatives with RE= Pr, Nd and Sm [28] exist. Their structures can be considered as CaBe₂Ge₂ superstructures with ordered vacancies [13].

Since our investigation mainly focuses on the magnetic and ¹¹⁹Sn Mössbauer spectroscopic characterization of the stannides RE₃Au₆Sn₅ (RE= La, Ce, Pr, Nd, Sm) and since the crystal chemistry of the prototype has been discussed in detail [11, 28], herein we only briefly discuss the structural peculiarities of Ce₃Au₆Sn₅.

A view of the Ce₃Au₆Sn₅ structure approximately along the b axis is presented in Fig. 1. Comparison of the positional parameters of the RE₃Au₆Sn₅ stannides with those of Ce₃Pd₆Sb₅ shows one distinct difference. The 2a Sn3 sites of the stannides all have higher z values than the antimonides [11, 28]. This means a change in the coordination sphere of the Sn3 atoms as compared to Sb3 in Ce₃Pd₆Sb₅ [11]. Consequently one should call these structures rather isopointal [29, 30] than isotypic. The [Au₆Sn₅] polyanion is three-dimensional in Ce₃Au₆Sn₅, while in the Ce₃Pd₆Sb₅ structure, there is no bonding of Sb3 to the adjacent layer (see Fig. 2 in [13]).

Fig. 1:

View of the Ce₃Au₆Sn₅ structure approximately along the b axis. Cerium, gold, and tin atoms are drawn as medium grey, blue, and magenta circles, respectively. The three-dimensional [Au₆Sn₅] network is emphasized. The two crystallographically independent cerium sites are indicated.

Fig. 2:

Coordination of the gold and tin atoms in Ce₃Au₆Sn₅. Cerium, gold, and tin atoms are drawn as medium grey, blue, and magenta circles, respectively. The site symmetries are indicated.

The shortest interatomic distances in the Ce₃Au₆Sn₅ structure occur for Au–Sn (266–320 pm). They are even slightly shorter than the sum of the covalent radii of 274 pm [31], indicating substantial Au–Sn bonding within the polyanionic network. The range of Au–Sn distances is similar to that of other ternary gold stannides, e.g., CeAuSn (280 pm) [32], Eu₂Au₂Sn₅ (272–301 pm) [33], EuAuSn (276–310 pm) [34] or Er₂Au₂Sn (307 pm) [35].

Besides Au–Sn bonding, the [Au₆Sn₅] polyanion is further stabilized by Au–Au bonding (aurophilic interactions). Each of the four crystallographically independent gold atoms has at least one short Au–Au distance. The various Au–Au distances range from 284 to 301 pm, close to that in fcc gold (288 pm) [36]. The importance of T–T bonding in such BaAl₄ related structures has been discussed in detail on the basis of electronic structure calculations [37, 38].

The strong shift (positional displacement) of the Sn3 sites (as compared to the antimonides) leads to the formation of Sn3–Sn1 bonds with a bond length of 320 pm. This Sn–Sn distance is comparable to that in the β-Sn structure (4 × 302 and 2 × 318 pm) [36] and such Sn–Sn distances typically occur in tin-rich stannides [39].

Finally we need to discuss the gold-tin coloring within the three-dimensional [Au₆Sn₅] networks. The crystals of the cerium and the neodymium compound revealed complete Au–Sn ordering while a substantial degree of Au/Sn mixing was observed for the 2b Au2 site of the samarium compound. Also small differences in the lattice parameters (Table 1) between our data and those published previously [14–16] most likely derive from inhomogeneities in the bulk samples.

3.2 Magnetic properties

Magnetic investigations have been performed for RE₃Au₆Sn₅ with RE= Ce, Pr, Nd, Sm and are represented in Figs. 3 and 4. Table 5 lists the determined properties and the fitting results. Except for Sm₃Au₆Sn₅, which exhibits the typical van Vleck paramagnetism, all compounds show Curie–Weiss behavior. Fitting the reciprocal susceptibility data leads to effective magnetic moments μ_eff that fit very well to the theoretical values of the respective free RE³⁺ ions. The small effective magnetic moment of Sm₃Au₆Sn₅ can be explained by the van Vleck paramagnetism and will be discussed below in more detail. A negative Weiss constant, which indicates dominant antiferromagnetic interactions in the paramagnetic range, could only be obtained for Sm₃Au₆Sn₅. The Weiss constants of Ce₃Au₆Sn₅ and Nd₃Au₆Sn₅ are almost zero, while a positive value (θ_p = 11(2) K) could be observed for Pr₃Au₆Sn₅. An antiferromagnetic transition could only be obtained for Sm₃Au₆Sn₅. Consequently, the Weiss constants are in accordance with the determined magnetic behavior, however, Pr₃Au₆Sn₅ shows no ferromagnetic ordering.

Fig. 3:

Magnetic properties of Ce₃Au₆Sn₅: Temperature dependence of the magnetic susceptibility (χ and χ⁻¹ data) measured at 10 kOe. The inset displays the magnetization isotherms at 3, 10 and 50 K.

Fig. 4:

Magnetic properties of Sm₃Au₆Sn₅: (top) temperature dependence of the magnetic susceptibility (χ and χ⁻¹ data) measured at 10 kOe. The inset presents the magnetic susceptibility in zero-field-cooled/field-cooled (ZFC/FC) mode at 1000 Oe in the low-temperature range; (middle) Magnetization isotherms at 3, 50 and 150 K; (bottom) Heat capacity measured in the temperature range of 2.1–300 K without an applied field. The inset shows the low-temperature area to highlight the singularity.

In Fig. 3 the magnetic properties of Ce₃Au₆Sn₅ are presented exemplarily for the compounds that show no magnetic ordering down to 2.5 K. The temperature dependence of the magnetic susceptibility and its inverse (χ and χ⁻¹ data) measured at 10 kOe is shown, and Curie–Weiss behavior can be clearly identified. The effective magnetic moment of μ_eff= 2.42(1) μ_B per Ce atom agrees well with the theoretical value of 2.54 μ_B for a free Ce³⁺ ion. Magnetic properties reported for the composition ‘CeAu₂Sn₂’ (≡CeAu₂Sn_1.66) show an effective magnetic moment of μ_eff= 2.46(1) μ_B per Ce [14]. Most likely in that publication the magnetic properties of Ce₃Au₆Sn₅ with small amounts of tin have been reported [14].

To ensure that no magnetic ordering takes place down to 2.5 K, low-field measurements with an external field strength of 100 Oe were performed in a zero-field and field-cooled (ZFC/FC) mode (not shown here) for all three compounds. The inset in Fig. 3 displays the magnetization isotherms of Ce₃Au₆Sn₅ measured at 3, 10 and 50 K. All isotherms are in line with paramagnetic character as reflected by the linear field dependence of the magnetization. At higher fields almost no tendency for saturation can be observed for the 3 K isotherm, and the magnetic moment at 3 K and 80 kOe [0.9(1) μ_B per Ce atom] is much lower than the expected saturation magnetization of 2.14 μ_B according to g_J × J. Such reduced magnetization values often occur in cerium intermetallic compounds [32, 40, 41] and can be attributed to crystal field splitting of the J= 5/2 magnetic ground state of Ce³⁺. Similar reduced values could be obtained for the praseodymium and neodymium compounds (see Table 5).

One particularity is the van Vleck paramagnetism typically shown by Sm compounds due to the proximity of the excited-state J= 7/2 multiplet to the ground-state J= 5/2 multiplet of the Sm³⁺ ions. The energy difference between these states is only about 1550 K, while the other angular momentum levels are correspondingly higher [42]. It has been proved that polycrystalline samples can be described by the equation

χM(T) = NAkB[μeff23(T − θp) + μB2δ],

where μ_eff is the effective magnetic moment, θ_p is the Weiss-constant, μ_B is the Bohr magneton, N_A is the Avogadro number and k_B is the Boltzmann constant. δ is an energy scale, which is defined as δ= 7 ΔE/20. The first term represents the Curie–Weiss susceptibility of the J= 5/2 ground state, while the second part represents the van Vleck susceptibility due to the small energy difference to the J= 7/2 multiplet [43]. A more detailed description can be found in the literature ([44] and references therein).

By using this equation the data could be described very well with the fitting parameters μ_eff= 0.70(1) μ_B, θ_p= −26(2) K and δ= 580(5) K (see Fig. 4). The effective magnetic moment is less than the 0.845 μ_B of the free ion for the J= 5/2 Hund’s rule ground state of Sm³⁺, and δ= 580(1) K corresponds to ΔE= 1657 K. This is somewhat higher than the 1550 K predicted by Stewart [42].

A heat capacity measurement of Sm₃Au₆Sn₅, depicted in the bottom panel of Fig. 4, confirms the existence of the magnetic ordering around 9.0(5) K. In the inset the low-temperature area is presented and no typical λ anomaly due to a broadened signal can be observed. As described above, the samarium single crystal exhibits Au/Sn mixing on one site leading to a distribution of different domains around the ideal composition resulting in a broadened signal.

3.3 ¹¹⁹Sn Mössbauer spectroscopy

¹¹⁹Sn spectra of RE₃Au₆Sn₅ (RE = La–Nd, Sm) are presented in Fig. 5 together with transmission integral fits. The corresponding fitting parameters are listed in Table 6. The spectra taken at ambient temperature could be well reproduced by a superposition of three resonances exhibiting isomer shifts in the range typical for tin in polyanionic networks of intermetallic compounds [between 1.73(1) and 2.28(1) mm·s⁻¹] with strongly different quadrupole splittings. In order to ensure the presence of tin sites having a distinctly different electronic environment, theoretical electric field gradient calculations were carried out for the Ce and Nd compounds (Table 7). As a result one obtains two more or less similar electronic surroundings for Sn1 and Sn3. The third tin site – Sn2 – shows a significantly weaker electric field gradient. This fact does not go along with a simple comparison of the nearest neighbor geometries (Fig. 2) and the site symmetries for the different tin atoms. Keeping this observation in mind the recorded spectra can be fitted by using three resonances by constraining the transmission integral ratios of Sn1:Sn2:Sn3 to 40:40:20 according to the multiplicities of the respective tin sites. Furthermore the calculated values for C_Q and η_Q can be directly converted into quadrupole splitting values (Table 7) and give a hint to the start of the fitting procedure. Finally a fit with 85 % of the calculated value for ΔE_Q led to the best agreement with the measured spectra, and in order to obtain consistent results, this variable was fixed, too. In the cases of the La, Pr and Sm compounds the values for the Ce and Nd compounds were extra- or interpolated. The calculations and the experimental fits reveal that for Sn1 and Sn2 the quadrupole splittings remain independent of the nature of the rare earth ion, whereas for Sn3 the quadrupole splitting decreases systematically with decreasing cation size. Regarding the deviation of the theoretical ΔE_Q value from the experimental one it should be kept in mind that the literature value of the ¹¹⁹Sn excited state nuclear electric quadrupole moment – which is essential for calculating C_Q – has been fluctuating during the past decades between a value of 8 fm² [45] and the currently accepted value of 13.2(1) fm² [27] and must be considered a quantity with an unknown degree of accuracy.

Fig. 5:

Experimental (data points) and simulated (continuous lines) ¹¹⁹Sn Mössbauer spectra of RE₃Au₆Sn₅ (RE = La–Sm) at ambient temperature.

Although the single crystal data of the samarium compound indicated substantial Au/Sn mixing (vide supra), the ¹¹⁹Sn spectrum of Sm₃Au₆Sn₅ gives no hint for substantial disorder in the bulk sample, also in comparison to the other spectra. The only detail that could be mentioned is the slightly broadened experimental line width. Similar Au/Sn mixing as found in the single crystal investigation of Sm₃Au₆Sn₅ has also been reported for other CaBe₂Ge₂ related stannides like RET₂Sn₂ (RE= La, Ce, Sm; T= Ni, Cu) [46], SrAu₂Sn₂ [17] and EuT₂Sn₂ (T= Pd, Pt, Au) [18]. In these systems the disorder had a great influence on the transmission integral ratios and could therefore be identified to be a bulk phenomenon.

In addition to room temperature experiments, measurements at 78 and 5 K were carried out for the La compound (not shown here) in order to study temperature dependent effects. No significant differences in isomer shift, quadrupole splitting and signal ratios could be detected. Based on these results, we conclude that the three Sn sites have similar Lamb-Mössbauer factors, thus validating the above analysis procedure.