Diterbium heptanickel: 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 intergrowth 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 icosahedra 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.

The crystal structure of the title compound, Tb 2 Ni 7 , 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. [419][420][421][422][423][424][425][426][427][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 Tb 1.962 (4) Ni 7 . The title compound adopts the Ce 2 Ni 7 structure type and can also be derived from the CaCu 5 structure type as an intergrowth 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 icosahedra 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.
Supporting information for this paper is available from the IUCr electronic archives (Reference: FJ2679).

Comment
A lot of works have been published about R 2 Ni 7 stoichiometry compounds (R = rare earth element) (see, Lemaire et al., 1967;Lemaire & Paccard, 1969;Virkar & Raman, 1969;Buschow & van der Goot, 1970) with either β-Gd 2 Co 7 (Bertaut et al., 1965) or Ce 2 Ni 7 (Cromer & Larson, 1959) structure types. According to Virkar & Raman (1969) the hightemperature modifications adopt the rhombohedral β-Gd 2 Co 7 type structure whereas the low-temperature phases are isomorphic with Ce 2 Ni 7 . On the other hand, Lemaire et al. (1967) observed the coexistence of both modifications even in annealed at 1373 K samples for Pr 2 Ni 7 , Nd 2 Ni 7 , Gd 2 Ni 7 , Tb 2 Ni 7 , and Dy 2 Ni 7 . Buschow & van der Goot (1970) investigated series of R 2 Ni 7 compounds and concluded the crystal structures of the R 2 Ni 7 compounds are dependent on the R atom size. The transformation between these two polymorphic forms is of a martensitic type.
Our research work mainly deals with the heavy rare earth -transition metal (R-T) systems. And the crystal structures of the compounds forming in such systems are of the most interest. Investigation of the Tb-Ni system at 1070 K resulted in good agreement with the literature data for unit cell parameters for all compounds obtained from powder X-ray diffraction using starting coordinates of appropriate structure types. It was noted there is no any information in literature about crystal structure refinement of heavy rare earth R 2 Ni 7 compounds. Our recent work was devoted to the refinement of the Dy 2 Ni 7 compound, which crystal stucture is isomorphous with β-Gd 2 Co 7 (see, Levytskyy et al., 2012). In present study the crystal structure of Tb 2 Ni 7 was redetermined with high accurracy using single-crystal X-ray method.
The structure is characterized by two independent terbium atom sites (both 4f Wyckoff positions) and five nickel atom sites (12k, 6h, 4f, 4e and 2a). The unit cell of diterbium heptanickel is shown in Fig. 1. The structure may be viewed as staking of RT 5 blocks corresponding to the CaCu 5 -type and R 2 T 4 blocks corresponding to the MgCu 2 -type structures. The presence of the same Kagome net in the structure types of CaCu 5 and the Laves phase MgCu 2 allows a combination of both structural motifs along the 6 3 screw axis giving an intergrowth structure: 4RT 5 + 2R 2 T 4 = 4R 2 T 7 (Parthé et al., 1985;Grin, 1992).
In Fig

Experimental
The sample was prepared from powdered commercially available pure elements: sublimed bulk pieces of terbium metal with a claimed purity of 99.9 at.% (Strem Chemicals) and 99.99% pure nickel powder (Aldrich Chem. Inc.). A mixture of the powders was compacted into a pellet. It was arc-melted under an argon atmosphere on a water-cooled copper hearth.
The alloy button (~1 g) was turned over and remelted three times to improve homogeneity. Subsequently, the sample was annealed in an evacuated silica tube for four weeks at 1070 K. Shiny metallic gray prysmatic shaped crystals were isolated mechanically from crushed sample with a help of microscope.