Three pnictides with Sm₆Rh₃₀Si₁₉-type structure: Sm₆Rh₃₀P₁₉, Lu₆Rh₃₀P₁₉ and Eu₆Rh₃₀Sb₁₉

1 Introduction

The rare earth (RE) transition (T) metal phosphorus systems with the 3d transition metals have intensively been studied and a manifold of ternary phosphides RE _x T _y P _z has been structurally characterized. The crystal chemical data are summarized in review articles ^{1 , 2} and in the Pearson data base. ³ This family of phosphides comprises phosphorus-rich phases with P₂ dumb-bells, compounds with extended [T _y P _z ] ^δ− polyanionic networks with substantial covalent T−P bonding and many metal-rich phases with isolated (no P–P bonding) phosphorus atoms in trigonal prismatic metal coordination. Several isothermal sections have been studied, e.g., La–Ni–P and Ce–Ni–P, ⁴ Y–Cu–P, ⁵ Ce–Cu–P ⁶ or Nd-Cu-P. ⁷

The RE-T-P systems with the 4d and especially the 5d transition metals have only scarcely been studied. This might be due to the lower reactivity of the heavier transition metals and also to the high price of the noble metals. So far only 25 RE _x Rh _y P _z phases have been reported, while the Ce–Ni–P system alone contains 15 Ce _x Ni _y P _z phases. ⁴ The reported RE _x Rh _y P _z phases are RERhP (RE = Sc, La, Ce), ^{8 , 9 , 10 , 11} RERh₂P₂ (RE = La, Ce, Pr, Nd, Eu), ^{12 , 13} RE ₂Rh₁₂P₇ (RE = Y, La, Nd, Gd, Tb, Dy, Ho, Er, Yb), ^{14 , 15 , 16} RE ₆Rh₃₂P₁₇ (RE = La, Ce), ¹⁷ RERh₆P₄ (RE = Sc, Yb, Lu), ¹⁸ RE ₆Rh₃₀P₁₉ (RE = Eu, Yb), ¹⁹ LaRh₄P₁₂ ²⁰ and La₄Rh₈P₉. ²¹

Some of the RE _x Rh _y P _z phosphides have been studied with respect to their magnetic and electrical transport properties. Equiatomic CeRhP ⁸ is a valence fluctuating compound with a Kondo temperature of around 400–500 K. The skutterudite phase LaRh₄P₁₂ was synthesized under high-pressure high-temperature conditions. ²⁰ LaRh₄P₁₂ shows a comparatively high transition to superconductivity at around 17 K. The transition temperatures of ScRhP (T _C = 2 K) ¹⁰ and LaRhP (T _C = 2.5 K) ⁹ are much lower. Surprisingly, the phosphide in this series, EuRh₂P₂, shows the unusual coexistence of a non-integral europium valence of 2.2 along with antiferromagnetic ordering below 50 K. ^{13 , 22 , 23 , 24 , 25}

Well-shaped single crystals of such phosphides can be grown under lead ²⁶ or bismuth flux ²⁷ conditions. Lead was used for the growth of Eu₆Rh₃₀P₁₉ and Yb₆Rh₃₀P₁₉ crystals, ¹⁹ while bismuth was found suitable for the RERh₆P₄ (RE = Sc, Yb, Lu) ¹⁸ crystals and La₄Rh₈P₉. ²¹

In continuation of the phase analytical studies of the RE-T-P systems, our bismuth flux experiments resulted in the new rhodium-rich phosphides Sm₆Rh₃₀P₁₉ and Lu₆Rh₃₀P₁₉ which adopt the Sm₆Rh₃₀Si₁₉ type ²⁸ structure. The crystal chemical details of both phosphides are reported herein. The isotypic antimonide Eu₆Rh₃₀Sb₁₉ was reported by Jeitschko et al. ²⁹ We have resynthesized a polycrystalline sample of this phase and investigated its magnetic properties and Mössbauer spectroscopic behavior.

2 Experimental

3 Structure refinements

The data sets of Sm₆Rh₃₀P₁₉ and Lu₆Rh₃₀P₁₉ showed hexagonal lattices with low Laue symmetry and the systematic extinctions 000l with l = 2n, leading to the possible space groups P6₃ and P6₃/m, of which the centrosymmetric one was chosen. The starting atomic parameters were deduced with the charge-flipping algorithm ³⁷ implemented in Superflip, ³⁸ and both structures were refined on F ² with the Jana2020 software package, ^{39 , 40} with anisotropic displacement parameters for all sites. Initially, the P4 atoms were placed on the 2b sites; however, they showed strongly enhanced U ₃₃ values and lower occupancy. In parallel to the work of Wurth et al. ¹⁹ on the related phosphides Ca₆Rh₃₀P₁₉, Eu₆Rh₃₀P₁₉ and Yb₆Rh₃₀P₁₉, we introduced a split position 4e with 25 % occupancy, which was refined with an isotropic displacement parameter (the crystal chemical consequences are discussed below). Separate refinement of the occupancy parameters of the remaining sites gave no hints for deviations from the ideal compositions. The final difference Fourier synthesis was contourless. Further details on the refinements, the positional, the displacement parameters and the interatomic distances (exemplarily listed for Sm₆Rh₃₀P₁₉) are presented in Tables 1–3.

CCDC 2443532 (Sm₆Rh₃₀P₁₉) and CCDC 2443531 (Lu₆Rh₃₀P₁₉) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

4 Crystal chemistry

The rhodium-rich phosphides Sm₆Rh₃₀P₁₉ and Lu₆Rh₃₀P₁₉ crystallize with a slightly modified variant of the Yb₆Co₃₀P₁₉ type structure, ⁴¹ similar to Eu₆Rh₃₀P₁₉ and Yb₆Rh₃₀P₁₉. ¹⁹ The crystal chemistry of the prototype has been discussed in detail by Jeitschko and Jakubowski-Ripke. ⁴¹ In the following discussion we focus on the differences observed for the respective rhodium compounds. Our discussion exemplarily relies on the Sm₆Rh₃₀P₁₉ structure.

A view of the Sm₆Rh₃₀P₁₉ structure along the c axis is presented in Figure 1. The rhodium and phosphorus atoms build a three-dimensional covalently bonded [Rh₃₀P₁₉] network. This is in agreement with the course of the interatomic distances. The Rh–P distances range from 225–255 pm, comparable to the sum of the covalent radii ⁴² of 235 pm for Rh + P. Comparable ranges of Rh–P distances have been observed for the closely related structures of ScRh₆P₄ (228–258 pm) ¹⁸ and La₄Rh₈P₉ (229–254 pm). ²¹ The bonded [Rh₃₀P₁₉] network is additionally stabilized by Rh–Rh bonding. The five crystallographically independent rhodium sites show Rh–Rh distances in the range from 276 to 288 pm, only slightly longer than in fcc rhodium (12 × 269 pm). ⁴³ This is the typical bonding pattern in the large family of metal-rich phosphides. ⁴⁴

Figure 1:

View of the Sm₆Rh₃₀P₁₉ structure along the c axis. Samarium, rhodium and phosphorus atoms are drawn as medium grey, blue and magenta circles, respectively. The three-dimensional [Rh₃₀P₁₉] network and the octahedral coordination of the P4 atoms (extending along the c axis) are emphasized.

The [Rh₃₀P₁₉] network leaves hexagonal prismatic cavities that are filled by the larger samarium atoms. The rectangular faces of these prisms are all capped by additional rhodium atoms, leading to coordination number 18, a so-called hexa-capped hexagonal prism. This type of rare earth coordination is frequently found in metal-rich rare earth transition metal phosphides. An overview is given in the review by Kuz’ma and Chykhrij. ¹

Another striking structural motif that is evident from Figure 1 is the infinite row of face-sharing P4@Rh5₆ octahedra that extends along the c axis. Figure 2 shows a cutout of such a row. This substructure is responsible for the crystal chemial differences with respect to the prototype Yb₆Co₃₀P₁₉. ⁴¹ The short c axis of the Sm₆Rh₃₀P₁₉ structure does not allow for occupancy of every octahedral void, since this would result in too short P–P distances of 191.7 pm (half the c axis). A way out of this dilemma would be a symmetry reduction to the non-centrosymmetric space group P 6 ‾ , where the 2b site splits into two one-fold sites of which only one is occupied. Also, all other sites split into two sites in the space group with lower symmetry. A stable refinement of this fully ordered model was obtained for the U₆Fe₃₀P₁₉ structure, ⁴⁵ while during the refinement of the Yb₆Co₃₀P₁₉ structure the displacement parameters of the atom pairs had to be constrained due to too strong correlations. Similar to our refinements, the structures of Ca₆Rh₃₀P₁₉, Eu₆Rh₃₀P₁₉ and Yb₆Rh₃₀P₁₉ ¹⁹ and the silicide Sm₆Rh₃₀Si₁₉ ²⁸ showed no long-range order of the phosphorus (silicon) atoms, but the formation of ordered microdomains. Thus, the structures were refined with a partially occupied split position 0 0 z in the higher symmetric space group P6₃/m. These microdomain problems with refinements in space groups P6₃/m (Ho₂Rh₁₂As₇ type) versus P 6 ‾ (Zr₂Fe₁₂P₇ type) are also encountered for the many pnictides of the family of 2-12-7 intermetallics. ^{15 , 46 , 47 , 48} Figure 2 shows the partially occupied P4 sites viewed along the c axis. Only the longer P–P distances which are at least equal or larger than the P–P single-bond distance of 223 pm can be considered. ⁴⁴ Keeping the multiplicity (and 25 % occupancy) of the 4e site in mind, only every fourth position can/needs to be occupied. Comparable ordering patterns have been reported for the silicides La₆Ni_1.54Si₃, La₅Ni_1.75Si₃, La₁₅Ni_6.62Si₁₀ and La₂₁Ni_10.49Si₁₅. ⁴⁹

Figure 2:

Cutout of the Sm₆Rh₃₀P₁₉ structure showing the strands of face sharing P4@Rh5₆ octahedra that extend along the c axis. Rhodium and phosphorus atoms are drawn as blue and magenta circles, respectively. The different distances (pm) induced by the split refinement are indicated. Note that this 4e site for the P4 atoms is a split position with only 25 % occupancy. In a chemically reasonable ordered model, only P–P distances of ≥253 pm are realized. For details see text.

The phosphorus atoms P1, P2 and P3 have trigonal prismatic metal coordination (Figure 3). For both, P2 and P3, the rectangular faces of the trigonal prisms are capped by additional rhodium atoms, leading to coordination number nine in the form of tricapped trigonal prisms, the typical phosphorus coordination in metal-rich phosphides with isolated (i.e., no P–P bonding) P atoms. ⁴⁴ Three trigonal prisms filled with the P1 atoms form compact units by sharing common edges. This leads to an emtpy Rh₆ prism, shaded in yellow color in Figure 3. These Rh₆ prisms are too small to be occupied by an additional atom. The central atom would have much too short distances (172 pm) to the P1 atoms. Geometrically one can describe the Sm₆Rh₃₀P₁₉ structure with a shamrock-like condensation pattern of trigonal, P-centered prisms. An overview of these shamrock-like building units is given by Zaremba et al. ⁵⁰ and a general classification of structures with condensed trigonal prisms as building units was published by Gladyshevskii and Grin. ⁵¹

Figure 3:

Projection of the Sm₆Rh₃₀P₁₉ structure along the short unit cell axes. Samarium, rhodium and phosphorus atoms are drawn as medium grey, blue and magenta circles, respectively. All atoms (except P4) lie on mirror planes at x = 1/4 and 3/4, accentuated by thin and thick lines. The trigonal prisms around the phosphorus atoms are highlighted. Atom designations are given at the lower right-hand part. The blue grey circle emphasizes the row of condensed P4@Rh5₆ octahedra shown in Figure 2 and the empty Rh₆ prisms are shaded in yellow.

Finally, we turn to the isotypic antimonide Eu₆Rh₃₀Sb₁₉. ²⁹ The structure of the europium compound was studied on the basis of single-crystal X-ray diffractometer data in space group P6₃/m and showed the same microdomain problems as Sm₆Rh₃₀P₁₉ and Lu₆Rh₃₀P₁₉. We have re-synthesized a polycrystalline sample of Eu₆Rh₃₀Sb₁₉ for physical property studies which are reported in the following.

5 Magnetic properties of Eu₆Rh₃₀Sb₁₉

The course of the cell volumes of the RE ₆Rh₃₀Sb₁₉ series ²⁹ showed a strong positive deviation for Eu₆Rh₃₀Sb₁₉, indicating at least partially divalent europium. We have therefore performed magnetic susceptibility measurements in order to clarify the magnetic ground state of Eu₆Rh₃₀Sb₁₉. The temperature dependence of the magnetic susceptibility of Eu₆Rh₃₀Sb₁₉ measured at 10 kOe is presented in Figure 4. The compound shows Curie-Weiss behavior above 25 K. A fit of the data (25–300 K range) with the Curie-Weiss law yielded an experimental magnetic moment of 7.78(1) µ_B Eu atom⁻¹ and a Weiss constant of −9.6(1) K. The experimentally derived moment is slightly smaller than that of the free ion value of Eu²⁺ (7.94 µ_B). ⁵² Such a slight moment reduction has been observed for a variety of intermetallic europium compounds. ^{53 , 54 , 55} The negative Weiss constant is indicative of antiferromagnetic interactions in the paramagnetic range.

Figure 4:

Temperature dependence of the magnetic susceptibility (χ and χ ⁻¹ values) of Eu₆Rh₃₀Sb₁₉ measured at 10 kOe.

The low-field measurement at 100 Oe (Figure 5) shows an anomaly in the low-temperature regime, but without a clear sign for antiferromagnetism. We have then measured the temperature dependence of the heat capacity of Eu₆Rh₃₀Sb₁₉. A clear λ-type anomaly is evident in the low-temperature regime (Figure 6), manifesting the magnetic ordering temperature of 3.9(4) K. The nature of magnetic ordering becomes clear from the magnetization measurements (Figure 7). At 10 and 50 K we observe a linear increase of the magnetization with increasing field as is expected for a paramagnetic material. The 4 K isotherm shows a weak deviation from a linear increase at around 40 kOe, most likely resulting from a spin-flip transition, and this underpins the antiferromagnetic ground state. The magnetization at 4 K and 80 kOe amounts to 5.33(1) µ_B Eu atom⁻¹, lower than the theoretical value of 7 µ_B for Eu²⁺. ⁵² Thus, much higher fields are necessary to achieve full parallel spin alignment.

Figure 5:

Low-field magnetic susceptibility of Eu₆Rh₃₀Sb₁₉ measured at 100 Oe in zero-field cooling and field-cooling mode.

Figure 6:

Heat capacity of Eu₆Rh₃₀Sb₁₉ measured without external field. The low-temperature behavior is shown in the insert.

Figure 7:

Magnetization isotherms of Eu₆Rh₃₀Sb₁₉ measured at T = 5, 10 and 50 K.

6 Mössbauer spectroscopic characterization of Eu₆Rh₃₀Sb₁₉

The ¹⁵¹Eu Mössbauer spectra of Eu₆Rh₃₀Sb₁₉ are shown in Figure 8 along with transmission integral fits. The underlying fitting parameters are summarized in Table 4. The Mössbauer spectra fully confirm the magnetic data. Eu₆Rh₃₀Sb₁₉ has a stable divalent ground state. The isomer shift (78 K data) of −10.44(1) mm s⁻¹ is within the usual range observed for divalent, predominantly covalently bonded intermetallic europium compounds. ^{56 , 57} The pnictide Zintl phases EuCd₂ X ₂ (X = P, As, Sb) exbibit a higher degree of ionic bonding and this is expressed in a more negative isomer shift of around −11.7 mm s⁻¹. ⁵⁸ Evidence for a tiny amount of trivalent europium is observed in the 78 K spectrum. The signal around 0 mm⁻¹ was included as simple Lorentzian in the fit. This small Eu(III) quantity most likely arises from surface oxidation of the sample. The non-cubic site symmetry (m..) of the europium atoms is expressed in a quadrupole splitting parameter of around 2.5 mm s⁻¹. The experimental line widths are in the usual range. The signal observed at 4.2 K (very close to the magnetic ordering temperature) is slightly broadened. The onset of magnetic ordering results in a hyperfine field splitting parameter of 2.7(4) T. Full magnetic hyperfine field splitting will occur at lower temperatures (not accessible with the used experimental setup).

Figure 8:

Experimental (dots) and simulated (continuous line) ¹⁵¹Eu Mössbauer spectra of Eu₆Rh₃₀Sb₁₉ at T = 4.2, 78 and 298 K.

Although the Eu₆Rh₃₀Sb₁₉ structure contains four crystallographically independent antimony sites, the ¹²¹Sb Mössbauer spectrum of the compound (Figure 9) was well reproduced with a single signal at an isomer shift of −7.12(2) mm s⁻¹. This value clearly manifests the antimonide character. ^{59 , 60 , 61 , 62} All antimony atoms have a very similar environment and – given the high natural line width of antimony – the observed signal is the envelope of the four sub-signals which cannot be resolved experimentally.

Figure 9:

Experimental (dots) and simulated (continuous line) ¹²¹Sb Mössbauer spectrum of Eu₆Rh₃₀Sb₁₉ at T = 78 K.