Osmium and magnesium: structural segregation in the rare earth-rich intermetallics RE 4OsMg (RE=La–Nd, Sm) and RE 9 TMg4 (RE=Gd, Tb)

Abstract

Abstract Ternary rare earth metal-rich intermetallic phases containing osmium and magnesium were obtained by induction melting of the elements in sealed niobium ampoules under argon followed by annealing in muffle furnaces. The large rare earth elements form the series of Gd4RhIn-type (F4̅3m) intermetallics RE 4OsMg with RE = La–Nd and Sm, while the smaller rare earth metals gadolinium and terbium form the Y9CoMg4-type (P63/mmc) phases Gd9OsMg4 and Tb9OsMg4. All samples were characterized by X-ray powder diffraction (Guinier technique). The structures of Ce4Os0.973Mg1.027 (a = 1406.54(7) pm, wR2 = 0.0478), Nd4Os0.978Mg1.022 (a = 1402.00(7) pm, wR2 = 0.0463), Sm4Os0.920Mg1.080 (a = 1387.33(5) pm, wR2 = 0.0378) and Gd9OsMg4 (a = 971.01(5), c = 980.43(5) pm, wR2 = 0.0494) were refined from single-crystal X-ray diffractometer data. The three RE 4OsMg phases show small degrees of Os/Mg mixing, as is frequently observed for Rh/In in Gd4RhIn-type intermetallics. The basic building units in both structures are osmium-centered RE 6 trigonal prisms that are condensed with empty RE 6 octahedra. The magnesium atoms in both types build Mg4 tetrahedra. The latter are isolated (312 pm Mg–Mg in Ce4OsMg) and incorporated within the three-dimensional network of prisms and octahedra in the RE 4OsMg phases while one observes rows of corner- and face-sharing tetrahedra in Gd9OsMg4 (305 and 314 pm Mg–Mg). In both structure types direct Os–Mg bonding is not observed.