Osmium and magnesium: structural segregation in the rare earth-rich intermetallics RE₄OsMg (RE=La–Nd, Sm) and RE₉TMg₄ (RE=Gd, Tb)

1 Introduction

Osmium is one of the high-melting and chemically very inert transition metals. This is reflected in the binary phase diagrams. So far no binary compounds have been reported with magnesium, zinc, cadmium, indium, thallium, lead and bismuth [1]. It is obvious, that this concerns mainly the low-melting main group metals as well as zinc and cadmium. Such empty binary phase diagrams have a large influence on the ternary ones. It is empirically known that the ternary phase diagrams mostly exhibit only few phases if the binary diagrams show few or none and vice versa.

The ternary phase diagrams RE-T-Mg and RE-T-Cd (RE=rare earth metal; T=electron-rich transition metal) contain a huge number of compounds [2], [3]; however, so far no representatives with osmium were known. Recent phase analytical studies in these systems with osmium revealed a series of ternary phases RE₄OsCd (RE=La, Ce, Pr, Nd, Sm, Gd) [4], RE₉OsMg₄ (RE=Y, Dy, Ho, Er, Tm, Lu) [5] and RE₁₀OsCd₃ (RE=Y, Sm, Gd–Tm, Lu) [4]. There seems to be a simple crystal chemical feature that enables formation of such ternary osmium-based compounds. In these rare earth-rich phases osmium and magnesium (cadmium) are separated in different polyhedra: osmium within trigonal rare earth prisms besides tetrahedral Cd₄ or triangular Cd₃ clusters, respectively rows of corner- and face-sharing Mg₄ tetrahedra. These magnesium and cadmium substructures are rare structural motifs in both solid state and molecular compounds.

The missing magnesium-based representatives with the light rare earth metals have now been synthesized and structurally characterized. Herein we report on the Gd₄RhIn-type series RE₄OsMg with RE=La, Ce, Pr, Nd, Sm and the Y₉CoMg₄-type phases Gd₉OsMg₄ and Tb₉OsMg₄.

2 Experimental

3 Crystal chemistry

The present phase analytical studies in the RE-Os-Mg systems complete our work on rare earth-rich magnesium intermetallics. The RE₄OsMg phases with RE=La–Nd and Sm crystallize with the cubic Gd₄RhIn-type structure [12], a ternary ordered version of the Ti₂Ni type [17]. The unit cell volume decreases from the lanthanum to the samarium compound, a consequence of the lanthanide contraction. An anomaly is observed for Ce₄OsMg. Its cell volume is even slightly smaller than that of Pr₄OsMg, indicating intermediate-valent cerium. This is similar to the isotypic series of RE₄OsCd phases [4] and their ruthenium-based counterparts RE₄RuMg [18], [19] and RE₄RuCd [20].

The crystal chemistry of the many RE₄TX (X=Mg, Cd, Al, In) phases has been reviewed in detail [2], [3]. Herein we only focus on the peculiarities of the RE₄OsMg series, exemplarily for Ce₄OsMg. This structure has two striking motifs that can be considered as basic building units. The osmium atoms fill trigonal cerium prisms with Ce–Os distances of 287–289 pm. The small range of distances arises from the fact that the osmium atoms are slightly displaced from the centers of the trigonal prisms. The Ce–Os distances compare well with the sum of the covalent radii [21] of 291 for Ce+Os, indicating substantial covalent Ce–Os bonding. The Ce₆Os trigonal prisms are capped by three further cerium atoms at 354 pm on the rectangular faces. Such tricapped trigonal prisms are a frequent coordination motif in intermetallic structures. As is emphasized in two review articles [2], [3], these Ce₆Os trigonal prisms are condensed via common edges to a three-dimensional network. Figure 1 shows the substructure of condensed Ce₆Os trigonal prisms. Since the Ce₄OsMg structure derives from the aristotype Ti₂Ni which adopts the diamond space group (vide infra), the tetrahedral symmetry is also preserved in the substructure of condensed prisms.

Fig. 1:

The substructure of condensed Os@Ce₆ trigonal prisms of Ce₄OsCd.

The second basic building units are Mg₄ tetrahedra (Fig. 2) which fill the cavities formed by the network of condensed prisms. These tetrahedra have ideal symmetry with 312 pm Mg–Mg distances, which are even slightly shorter than in hcp magnesium (6×320 and 6×321 pm) [22]. Such magnesium tetrahedra are a rare structural motif in solid state and molecular materials. Since the Mg₄ tetrahedra are embedded within the prismatic network, their nearest neighbors are exclusively cerium atoms. We thus observe a clear structural segregation of the magnesium and osmium atoms, avoiding direct Os–Mg bonding.

Fig. 2:

The magnesium, respectively cadmium substructures of Ce₄OsMg, Gd₉OsMg₄ and Er₁₀RhCd₃ [32]. Atom designations and relevant interatomic distances are given in pm.

A general feature observed for the RE₄OsMg phases concerns the small degree of Os/Mg on one of the 16e sites. At first sight this seems to contradict a ternary ordered variant; however, this mixing is understandable in view of the aristotype Ti₂Ni. During the symmetry reduction (the complete Bärnighausen tree is published in Ref. [17]) from the centrosymmetric space group Fd3̅m to the non-centrosymmetric space group F4̅3m, the 32e Ni site of Ti₂Ni splits into two 16e sites which are occupied by osmium and magnesium in the RE₄OsMg phases. The Os/Mg mixing can be considered as a kind of residual disorder.

Gd₉OsMg₄ and Tb₉OsMg₄ complete the series of RE₉OsMg₄ phases [5]. We thus observe a switch from the Gd₄RhIn to the Y₉CoMg₄ type between the rare earth elements samarium and gadolinium. This is the general trend for all RE₄TX series which exclusively form with the larger rare earth elements [2], [3]. Besides Y₉CoMg₄, the structure types Pr₂₃Ir₇Mg₄ [23], La₁₅Rh₅Cd₂ [24] and Er₁₀FeCd₃ [25] are formed for rare earth metal-rich phases with the smaller rare earth atoms.

The RE₉TMg₄ phases crystallize with an ordered version of the aristotype Co₂Al₅ [26], [27], [28], first observed for Y₉CoMg₄ [29]. Besides the RE₉CoMg₄ (RE=Y, Dy–Tm, Lu) series [29], also the representatives RE₉TMg₄ (RE=Y, Dy–Tm, Lu; T=Ru, Rh, Os, Ir) are known [5]. The structure type itself has been discussed in detail in the original work. Herein we briefly discuss the structure of Gd₉OsMg₄ that has been refined from single-crystal X-ray diffractometer data. Similar to the RE₄OsMg phases, also in the Gd₉OsMg₄ structure we observe a strict segregation of osmium and magnesium. The osmium atoms fill trigonal Gd₆ prisms with 282 pm Gd–Os distances, compatible with strong covalent Gd–Os bonding. These Os@Gd₆ trigonal prisms (TP) are condensed with empty Gd₆ octahedra (O) in the sequence –T–O–O– forming rows that extend in z direction. These rows alternate with rows of corner- and face-sharing Mg₄ tetrahedra (Fig. 2). Consequently, the whole structure can be described as a simple rod-packing of these two building units. This is discussed and illustrated in detail in ref. [29]. The row of corner- and face-sharing Mg₄ tetrahedra reminds of the structure of the hexagonal Laves phases REMg₂ [30]. The Mg–Mg distances within these rows range from 305 to 314 pm, similar to the data for the isolated tetrahedra in the RE₄OsMg phases.

While the RE₄TMg and RE₄TCd phases are isotypic, there is a striking difference for the RE₉TMg₄ ones. With magnesium we observe formation of the rows of tetrahedra, while in the related cadmium-based phases the corner-sharing position in the rows is occupied by a rare earth element (Fig. 2), leading to the general composition RE₁₀TCd₃, characterized for T=Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt [4], [25], [31], [32]. Recently also some aluminides RE₁₀TAl₃ (RE=Y, Ho, Er, Tm, Lu; T=Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) [33], [34] were reported.

Thus we observe two different coloring variants of the Co₂Al₅ type, the RE₉TMg₄ compositions on the one and the RE₁₀TCd₃ respectively RE₁₀TAl₃ ones on the other hand. In view of the different bonding patterns within the rows of corner- and face-sharing tetrahedra, the RE₁₀TCd₃/RE₁₀TAl₃ and RE₉TMg₄ phases are rather isopointal [35], [36] than isotypic.

4 Magnetic behavior

The temperature dependence of the inverse magnetic susceptibilities of the Gd₉OsMg₄ and Tb₉OsMg₄ samples was only linear in the high-temperature regimes (T≥250 K). An evaluation of these parts according to the Curie-Weiss law led to experimental magnetic moments of 8.2(1) μ_B per Gd atom and 9.3(1) μ_B per Tb atom which approximately match with the free ion values of 7.94 μ_B for Gd³⁺ and 9.72 μ_B for Tb³⁺ [37]. The low-temperature regime was substantially influenced by small amounts (not detectable in the X-ray powder patterns) of GdMg and TbMg where the atoms are ordered ferromagnetically at T_C=120, respectively 81 K [38]. These ferromagnetic impurity phases overlap the paramagnetic regime and we observe several weak anomalies. It is well known that these CsCl-type phases form as by-products for most of these rare earth-rich phases of magnesium as well as cadmium. Furthermore, they show the formation of solid solutions REMg_1−xT_x and RECd_1−xT_x with the respective transition metal [1]. Consequently, a distribution of domains with slightly different (now ternary) compositions is observed and thus a distribution of ferromagnetically ordering domains with different Curie temperatures. This is exactly the problem for the Gd₉OsMg₄ and Tb₉OsMg₄ samples studied herein and a deeper interpretation of the magnetically ordered regime is not appropriate. The dependence of the Curie temperature on the composition of such CsCl-type phases was exemplarily studied for the solid solution GdRu_xCd_1−x which extends up to x≈0.25 and shows a continuous decrease of T_C from 258 K for GdCd to 61.5 K for GdRu_0.20Cd_0.80 [39], [40].