Redetermination of dysprosium trinickel from single-crystal X-ray data

The crystal structure of the title compound, DyNi3, was redetermined from single-crystal X-ray diffraction data. In comparison with previous studies based on powder X-ray diffraction data [Lemaire & Paccard (1969 ▶). Bull. Soc. Fr. Minéral. Cristallogr. 92, 9–16; Tsai et al. (1974 ▶). J. Appl. Phys. 45, 3582–3586], the present redetermination revealed refined coordinates and anisotropic displacement parameters for all atoms. The crystal structure of DyNi3 adopts the PuNi3 structure type and can be derived from the CaCu5 structure type as an intergrowth structure. The asymmetric unit contains two Dy sites (site symmetries 3m and -3) and three Ni sites (m, 3m and -3m). The two different coordination polyhedra of Dy are a Frank–Kasper polyhedron formed by four Dy and 12 Ni atoms and a pseudo-Frank–Kasper polyhedron formed by two Dy and 18 Ni atoms. The three different coordination polyhedra of Ni are Frank–Kasper icosahedra formed by five Dy and seven Ni atoms, three Dy and nine Ni atoms, and six Dy and six Ni atoms.

The crystal structure of the title compound, DyNi 3 , was redetermined from single-crystal X-ray diffraction data. In comparison with previous studies based on powder X-ray diffraction data [Lemaire & Paccard (1969). Bull. Soc. Fr. Minéral. Cristallogr. 92, 9-16;Tsai et al. (1974). J. Appl. Phys. 45,[3582][3583][3584][3585][3586], the present redetermination revealed refined coordinates and anisotropic displacement parameters for all atoms. The crystal structure of DyNi 3 adopts the PuNi 3 structure type and can be derived from the CaCu 5 structure type as an intergrowth structure. The asymmetric unit contains two Dy sites (site symmetries 3m and 3m) and three Ni sites (m, 3m and 3m). The two different coordination polyhedra of Dy are a Frank-Kasper polyhedron formed by four Dy and 12 Ni atoms and a pseudo-Frank-Kasper polyhedron formed by two Dy and 18 Ni atoms. The three different coordination polyhedra of Ni are Frank-Kasper icosahedra formed by five Dy and seven Ni atoms, three Dy and nine Ni atoms, and six Dy and six Ni atoms.

Crystal data
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2688).
The present work contains the results of the full single-crystal X-ray determination of DyNi 3 , including refinement of the atomic coordinates and the temperature factors for all atoms. These results confirm the belonging to the PuNi 3 structure type in space group R3m. A view of the crystal structure of DyNi 3 is shown in Fig. 1. As has been noted previously (Yakinthos & Paccard, 1972), the crystal structure of DyNi 3 can be derived from the CaCu 5 structure type (Haucke, 1940;Nowotny, 1942). It consists of stacks of RX 5 blocks (CaCu 5 -type) and R 2 X 4 blocks (MgCu 2 -type (Friauf, 1927;Ohba et al., 1984)). Both types have the same Kagome net of Ni atoms that allows a combination of both structural motifs along the 3-fold inversion axis. As a result, it can be considered as an intergrowth structure: R 2 X 4 + RX 5 = 3RX 3 (Parthé et al., 1985;Grin, 1992).

Experimental
The sample was prepared of the powdered 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 remolten three times to improve homogeneity. Subsequently, the sample was annealed in an evacuated silica tube under an argon atmosphere for four weeks at 1070 K. Shiny grey irregular-shaped crystals were isolated mechanically with a help of microscope by crushing the sample.

Refinement
The atomic positions found from the direct methods structure solution were in good agreement with those from the PuNi 3 structure type and were used as starting parameters for the structure refinement. The highest Fourier difference peak of 2.77 e Å -3 is at (0 0 0.1019) and 0.91 Å away from the Dy1 atom. The deepest hole of -1.33 e Å -3 is at (0.8263 0.9132 0.0194) and 0.88 Å away from the Dy2 atom. SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 1999); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).   The ab projection of the unit cell and coordination polyhedra for all types of atoms in the DyNi 3 structure where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 2.77 e Å −3 Δρ min = −1.33 e Å −3 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.