Didysprosium heptanickel

The title compound, Dy2Ni7, adopts the β-Gd2Co7-type structure type. The asymmetric unit contains two Dy sites (both site symmetry 3m) and five Ni sites (site symmetries .m, .2/m and -3m, and two 3m). The four different Ni coordination polyhedra are Frank–Kasper icosahedra formed by five Dy and seven Ni atoms, four Dy and eight Ni, three Dy and nine Ni, and six Dy and six Ni atoms, respectively. The two different Dy coordination polyhedra are either pseudo Frank–Kasper icosahedra formed by two Dy and 18 Ni atoms or normal Frank–Kasper icosahedra formed by four Dy and 12 Ni atoms.

The title compound, Dy 2 Ni 7 , adopts the -Gd 2 Co 7 -type structure type. The asymmetric unit contains two Dy sites (both site symmetry 3m) and five Ni sites (site symmetries .m, .2/m and 3m, and two 3m). The four different Ni coordination polyhedra are Frank-Kasper icosahedra formed by five Dy and seven Ni atoms, four Dy and eight Ni, three Dy and nine Ni, and six Dy and six Ni atoms, respectively. The two different Dy coordination polyhedra are either pseudo Frank-Kasper icosahedra formed by two Dy and 18 Ni atoms or normal Frank-Kasper icosahedra formed by four Dy and 12 Ni atoms.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: VN2032).
without specifying atomic coordinates. Buschow & van der Goot (1970) prepared a series of isotypic compounds with a composition close to R 2 Ni 7 (R = Y, La-Nd, Sm, Gd-Er) and from the X-ray powder diffraction data confirmed the lattice parameters for Dy 2 Ni 7 .
In this work we carried out a single-crystal investigation of the Dy 2 Ni 7 inter-metallic compound. A view of the crystal structure of Dy 2 Ni 7 is shown in Fig. 1. The structure belongs to the β-Gd 2 Co 7 structure type (Bertaut et al., 1965) and consists of stacks of RX 5 blocks corresponding to the CaCu 5 -type structure and R 2 X 4 blocks corresponding to the MgCu 2type structure. 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 3-fold inversion axis giving an inter-growth structure: 2RX 5 + R 2 X 4 = 2R 2 X 7 . The Kagome net serves as the common interface in the structure (Parthé et al., 1985;Grin, 1992).
In Fig. 2  The Dy1 atom is surrounded by 2 Dy atoms and 18 Ni atoms. The Dy2 atom is surrounded by 4 Dy atoms and 12 Ni atoms.

Experimental
The sample was prepared from the commercially available pure elements: sublimed bulk pieces of dysprosium metal with a claimed purity of 99.99 at.% (Alfa Aesar, Johnson Matthey) and electrolytic nickel (99.99% pure) piece (Aldrich). A mixture of the powders was compacted in stainless steel dies. The pellet 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 evacuated silica tube under an argon atmosphere for four weeks at 1070 K.
Shiny metallic gray plate-like single crystals were isolated mechanically by crushing the sample.

Refinement
The atomic positions found from the ab initio structure solution were in good agreement with those from the β-Gd 2 Co 7 structure type and were used as starting point for the structure refinement. The highest Fourier difference peak of 5.07 e Å -3 is at (1/3 2/3 0.0305) and 0.89 Å away from the Ni4 atom. The deepest hole (-2.37 e Å -3 ) is at (1/3 2/3 0.0984) and 1.49 Å away from the Ni1 atom. SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 1999); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).   Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.