Diterbium hepta-nickel: a crystal structure redetermination

The crystal structure of the title compound, Tb2Ni7, was redetermined from single-crystal X-ray diffraction data. In comparison with previous studies based on powder X-ray diffraction data [Lemaire et al. (1967). C. R. Acad. Sci. Ser. B, 265, 1280-1282; Lemaire & Paccard (1969). Bull. Soc. Fr. Mineral. Cristallogr. 92, 9-16; Buschow & van der Goot (1970). J. Less-Common Met. 22, 419-428], the present redetermination affords refined coordinates and anisotropic displacement parameters for all atoms. A partial occupation for one Tb atom results in the non-stoichiometric composition Tb1.962 (4)Ni7. The title compound adopts the Ce2Ni7 structure type and can also be derived from the CaCu5 structure type as an inter-growth structure. The asymmetric unit contains two Tb sites (both site symmetries 3m.) and five Ni sites (.m., mm2, 3m., 3m., -3m.). The two different coordination polyhedra of Tb are a Frank-Kasper polyhedron formed by four Tb and 12 Ni atoms and a pseudo Frank-Kasper polyhedron formed by two Tb and 18 Ni atoms. The four different coordination polyhedra of Ni are Frank-Kasper icosa-hedra formed by five Tb and seven Ni atoms, four Tb and eight Ni atoms, three Tb and nine Ni atoms, and six Tb and six Ni atoms, respectively.