Gd₅Rh₁₉Sb₁₂ − a metal-rich antimonide with a Sc₅Co₁₉P₁₂ related structure

1 Introduction

The ternary systems RE-T-P ¹ and RE-T-Si ^{2 , 3} (RE = rare earth element, T = electron-rich transition metal) have intensively been studied and a manifold of ternary phosphides and silicides has been structurally characterized. ⁴ The transition metal-rich parts of these phase diagrams are characterized by a large family of compounds with a metal-to-metalloid ratio of exactly or close to 2:1. The basic geometrical building units of all these structures are phosphorus or silicon centered trigonal prisms formed by the RE and/or T atoms. Condensation of these prisms leads to a variety of extended geometrical motifs which spread from isolated prisms to shamrock-like trimeric units up to larger arrangements which are usually called propeller-shaped assemblies. ^{5 , 6} This kind of modular principle is helpful to explain and classify the different, sometimes complex structure types.

Some of the structural families observed for the phosphides and silicides also occur with the higher homologues arsenic and germanium; however, to a lesser extent. While many of the RE-T-Ge systems have thoroughly been studied, ^{7 , 8} systematic phase analytical knowledge of the respective arsenic systems is poor. One of the largest series of isotypic representatives are the Zr₂Fe₁₂P₇-type phases RE₂(Co, Ni, Rh)₁₂P₇ and RE₂(Co, Ni, Rh)₁₂As₇. ⁴ In going to antimony, only the metal-rich series RE₃Pt₇Sb₄ (RE = Ce, Pr, Nd, Sm) ⁹ and RE₆Rh₃₀Sb₁₉ (RE = La–Nd, Sm, Eu) ¹⁰ with antimony centered trigonal prisms are known.

During phase analytical studies in the RE-T-Sb systems we have now obtained Gd₅Rh₁₉Sb₁₂, the first antimonide with a Sc₅Co₁₉P₁₂-related ¹¹ structure. In the field of intermetallics, Sc₅Co₁₉P₁₂ type phases are known for the phosphides A₅T₁₉P₁₂ (A = Ca, Zr, Hf, rare earth metal; T = Fe, Co, Ni, Cu, Ru, Rh and Ir), ^{12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27} the tetrelides (Sr, Ba, Eu, Yb)₅Mg₁₉(Si, Ge)₁₂ ^{28 , 29 , 30 , 31 , 32} and the recently reported silicides RE₅Ir₁₉Si₁₂ (RE = Y, Gd–Dy, Er, Tm, Lu). ³³ Gd₅Rh₁₉Sb₁₂ is a remarkable compound since phosphorus was substituted by the much larger homologue antimony and still no arsenides with this structure type are known. The synthesis and structural chemistry of Gd₅Rh₁₉Sb₁₂ are reported herein.

2 Experimental

3 Structure refinement

The diffractometer data set showed a hexagonal lattice and no further systematic extinctions. In agreement with our earlier studies on the RE₅Ir₁₉P₁₂ phosphides ²⁷ and RE₅Ir₁₉Si₁₂ silicides, ³³ the non-centrosymmetric space group P 6 ‾ 2m was tested first. The starting atomic parameters were deduced with the charge-flipping algorithm ³⁶ implemented in Superflip ³⁷ and the structure was refined on F ² with the Jana2020 software package, ^{38 , 39} with anisotropic displacement parameters for all sites, except the 2e Rh5 split position which was refined with an isotropic displacement parameter. Separate refinements of the occupancy parameters gave no hint for deviations from the ideal composition. The Rh5 site is the decisive difference to the Sc₅Co₁₉P₁₂-type, since these rhodium atoms are shifted by c/2.

The split position called for a symmetry reduction. The simplest step is a translationengleiche symmetry reduction of index 2 to the trigonal space group P31m, where the 2e site splits into two one-fold sites 1a, of which only one needs to be occupied. The corresponding Bärnighausen tree ^{40 , 41 , 42 , 43} is presented in Figure 1. We have thus transferred the coordinates of the subcell refinement to space group P31m and refined the structure again, now with anisotropic displacement parameters for all sites. The loss of the 6 ‾ axis leads to twin formation. In the final refinement the twin matrices (−1 0 0, −1 1 0, 0 0 –1) and (−1 0 0, 0 –1 0, 0 0 –1) were introduced and the refinement for the ordered model smoothly converged to the residuals listed in Table 1. The domain ratio of 0.44(14): 0.06(8): 0.44(8): 0.06(8) indicated, that the crystal essentially consisted of two equal domains only (no twinning by inversion; the small domains are in the order of the standard deviation). All atoms are located on sites 0 0 z, 1/3 2/3 z and x y z. Due to the floating origin ^{44 , 45} and to avoid large correlations we fixed the z parameter of the Sb1 position (the one with the highest total atomic form factor). This is similar to the structures of ScRh₆P₄ ⁴⁶ and YbMg_0.75In_1.25. ⁴⁷ The final difference Fourier analyses was contourless. Further details on the refinement, the atomic coordinates, the displacement parameters and the interatomic distances are listed in Tables 1–3.

Figure 1:

Group-subgroup scheme for the superstructure formation of Gd₅Rh₁₉Sb₁₂ in the Bärnighausen formalism. ^{40 , 41 , 42 , 43} The index for the translationengleiche symmetry reduction and the evolution of the atomic parameters are given.

CCDC–2443530 contains 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

Gd₅Rh₁₉Sb₁₂ is the first antimonide in the family of Sc₅Co₁₉P₁₂-related intermetallic compounds. The substitution of phosphorus by the larger antimony (110 vs. 141 pm covalent radius ⁴⁸ ) leads to a drastic increase of the unit cell parameters (a = 1,349.3(3), c = 417.99(12) pm), when compared with the gadolinium-based phosphides Gd₅Co₁₉P₁₂ (a = 1,210.04(3), c = 370.0(1) pm) ¹⁴ and Gd₅Ir₁₉P₁₂ (a = 1,262.9(3), c = 395.7(1) pm). ²⁷ The increase is even stronger for the tetrelides (Sr, Ba, Eu, Yb)₅Mg₁₉(Si, Ge)₁₂, ^{28 , 29 , 30 , 31 , 32} e. g., a = 1,479.3(1), c = 445.2(1) pm for Eu₈Mg₁₆Ge₁₂. ³⁰ This leads to a small but important difference with respect to the prototype Sc₅Co₁₉P₁₂. ¹¹ The crystal chemical differences are addressed in the following.

Before starting with the crystal chemical discussion, we first concentrate on the role of the Rh5 atoms, which manifest the difference to the Sc₅Co₁₉P₁₂ type. In the prototype structure, the corresponding cobalt atoms fill a trigonal Sc₆ prism at the unit cell origin, while in the case of Gd₅Rh₁₉Sb₁₂, the Rh5 atoms are shifted by c/2 to the Gd1₃ triangles; however, refined with a split position in the prototype space group symmetry, P 6 ‾ 2m. A symmetry reduction then leads to a fully ordered model. The group-subgroup relation for the superstructure formation is presented in the Bärnighausen formalism in Figure 1. The translationengleiche symmetry reduction of index 2 (P 6 ‾ 2m → P31m) leads to free z parameters for all atoms and a splitting of the 2e site (split position in the subcell refinement) into two 1a sites, of which only one is occupied in the ordered model. This corresponds to a shift of the Rh5 atoms away from the Gd1₃ triangle (loss of the two-fold axis and of the mirror plane perpendicular to the c axis). Considering this peculiar detail, Gd₅Rh₁₉Sb₁₂ crystallizes with its own structure type.

The Gd₅Rh₁₉Sb₁₂ structure contains five crystallographically independent rhodium sites. These rhodium atoms have between three and six nearest antimony neighbors with Rh–Sb distances ranging from 255–290 pm, comparable to the antimonides CeRhSb (268–287 pm Rh–Sb), ⁴⁹ Ce₂Rh₃Sb₄ (259–270 pm Rh–Sb) ⁵⁰ or Ce₈Rh₁₇Sb₁₄ (257–272 pm Rh–Sb). ⁵¹ All these Rh–Sb distances are close to the sum of the covalent radii ⁴⁸ of 266 pm for Rh + Sb. Together, the rhodium and antimony atoms build a covalently bonded [Rh₁₉Sb₁₂] network (Figure 2). This network is additionally stabilized by weak Rh–Rh bonding. The Rh–Rh distances in the Gd₅Rh₁₉Sb₁₂ structure range from 292–336 pm, all slightly longer than in fcc rhodium (12 × 269 pm). ⁵² The two crystallographically independent gadolinium atoms fill slightly distorted hexagonal prismatic cavities within the [Rh₁₉Sb₁₂] network.

Figure 2:

View of the Gd₅Rh₁₉Sb₁₂ structure along the c axis. Gadolinium, rhodium and antimony atoms are drawn as medium grey, blue and magenta circles, respectively. The [Rh₁₉Sb₁₂] network is emphasized. Atom designations are given at the lower right-hand part.

The structures of metal-rich phosphides can geometrically be described by a condensation pattern of phosphorus-centered trigonal prisms formed by the metal atoms. ⁵ Often the condensation patterns resemble a shamrock-like arrangement. ⁶ In analogy to the RE₅T₁₉P₁₂ phosphides ^{11 , 14 , 27} and RE₅Ir₁₉Si₁₂ silicides, ³³ we have drawn the Gd₅Rh₁₉Sb₁₂ structure with the trigonal prismatic arrangement around the antimony atoms (Figure 3). The shamrock-like motif that undulates through the middle of the unit cell looks quite regular with antimony atoms approximately within the centers of the trigonal prisms. This is different for the motif located around the origin of the unit cell. The three inner prisms that are directly condensed to form the Gd₆ prism have one strongly elongated edge and the antimony atoms are shifted towards the c axis. Also, the terminal prisms of this motif show a shift of the antimony atoms towards the outer Rh₄ faces. In Figure 4 we compare this arrangement with the one in the recently reported silicide Tm₅Ir₁₉Si₁₂, ³³ where all prisms are almost regular.

Figure 3:

Projection of the Gd₅Rh₁₉Sb₁₂ structure along the c axis. Gadolinium, rhodium and antimony atoms are drawn as medium grey, blue and magenta circles, respectively. All atoms lie on mirror planes at z = 0 (thin lines) and z = 1/2 (thick lines). The trigonal prismatic coordination of the antimony atoms in emphasized.

Figure 4:

Comparison of the Tm₅Ir₁₉Si₁₂ and Gd₅Rh₁₉Sb₁₂ structures. Thulium (gadolinium), iridium (rhodium) and silicon (antimony) atoms are drawn as medium grey, blue and magenta circles, respectively. The trigonal prismatic building units around the origins of the unit cells are shown as projections onto the ab planes in the upper part. The lower part shows the local trigonal prismatic coordination in the strands that extend in c direction. Atom designations and relevant interatomic distances are given.

The main difference between the Sc₅Co₁₉P₁₂-type phosphides and silicides with the comparatively small phosphorus and silicon atoms on one side and Gd₅Rh₁₉Sb₁₂ on the other concerns the position of the rhodium atoms on the c axis. The local coordination in the gadolinium prisms in Gd₅Rh₁₉Sb₁₂ is compared with that in Tm₅Ir₁₉Si₁₂ in the lower part of Figure 4. The Rh5 atoms in Gd₅Rh₁₉Sb₁₂ are shifted by ∼c/2 with respect to Ir4 in Tm₅Ir₁₉Si₁₂ and the Sb2 atoms move towards the c axis, forming the trigonal prisms around Rh5. The Gd1 atoms are then capping the rectangular faces of these trigonal prisms. The Sb2–Sb2 contact of 339 pm, i. e., the edge of the triangles, is a weak one and corresponds to the secondary Sb–Sb interactions in elemental antimony (3 × 291 and 3 × 336 pm Sb–Sb). ⁵² The distance of the Rh5 atoms to the Gd2 atoms of 286 pm is exactly the value of the sum of the covalent radii for Gd + Rh. ⁴⁸ Thus, the Rh5@Gd2₃ coordination is comparable to the Ir4@Si1₃ coordination in Tm₅Ir₁₉Si₁₂.

Summing up, Gd₅Rh₁₉Sb₁₂ adds a new structural motif to the family of the so-called 5-19-12 phases and can be considered as a proper structure type. The pnictides assigned to the Ho₅Ni₁₉P₁₂ type ⁵³ are those phases that show no split position for the atom at the origin, while those related to the Sc₅Co₁₉P₁₂ type ¹¹ show cobalt split positions as manifested by single crystal X-ray diffraction data also for Zr₅Co₁₉P₁₂ ¹³ and Ho₅Co₁₉P₁₂ ¹⁴ (these two branches are distinguished in the Pearson data base ⁴ ). Important parameters that have an impact on the T5 subcell site are the size of the rare earth element (one can play with the lanthanide contraction, i.e., the size of the rare earth element) and also the crystal growth conditions (kind of flux and the temperature profile). Especially the thermal treatment of the samples influences a potential long-range order. Thus, it is difficult to predict the correct description of a 5-19-12 crystal. Furthermore, Gd₅Rh₁₉Sb₁₂ is the first antimonide of this atomic arrangement with the much larger antimony atoms.

Th₅Fe₁₉P₁₂ ⁵⁴ and Yb₅Ni₁₉P₁₂, ⁵⁵ on the other side, have the same atomic ratios as Tm₅Ir₁₉Si₁₂ and Gd₅Rh₁₉Sb₁₂ but a different condensation pattern of their phosphorus-centered trigonal prisms.

Future studies must now focus on RE₅Rh₁₉Sb₁₂ antimonides with rare earth elements larger and smaller than gadolinium, in order to check the existence range of this new type and further on antimony-arsenic substitution, since this is a gap with respect to the many RE₅T₁₉P₁₂ phosphides.