Redetermination of Dy3Ni from single-crystal X-ray data

The classification of the title compound, tridysprosium nickel, into the Fe3C (or Al3Ni) structure type has been deduced from powder X-ray diffraction data with lattice parameters reported in a previous study [Lemaire & Paccard (1967 ▶). Bull. Soc. Fr. Mineral. Cristallogr. 40, 311–315]. The current re-investigation of Dy3Ni based on single-crystal X-ray data revealed atomic positional parameters and anisotropic displacement parameters with high precision. The asymmetric unit consists of two Dy and one Ni atoms. One Dy atom has site symmetry .m. (Wyckoff position 4c) and is surrounded by twelve Dy and three Ni atoms. The other Dy atom (site symmetry 1, 8d) has eleven Dy and three Ni atoms as neighbours, forming a distorted Frank–Kasper polyhedron. The coordination polyhedron of the Ni atom (.m., 4c) is a tricapped trigonal prism formed by nine Dy atoms.

The classification of the title compound, tridysprosium nickel, into the Fe 3 C (or Al 3 Ni) structure type has been deduced from powder X-ray diffraction data with lattice parameters reported in a previous study [Lemaire & Paccard (1967). Bull. Soc. Fr. Mineral. Cristallogr. 40,[311][312][313][314][315]. The current re-investigation of Dy 3 Ni based on single-crystal X-ray data revealed atomic positional parameters and anisotropic displacement parameters with high precision. The asymmetric unit consists of two Dy and one Ni atoms. One Dy atom has site symmetry .m. (Wyckoff position 4c) and is surrounded by twelve Dy and three Ni atoms. The other Dy atom (site symmetry 1, 8d) has eleven Dy and three Ni atoms as neighbours, forming a distorted Frank-Kasper polyhedron. The coordination polyhedron of the Ni atom (.m., 4c) is a tricapped trigonal prism formed by nine Dy atoms.

Comment
Lattice parameters for RE 3 Ni compounds with RE = Y, La, Pr, Nd, Sm, Gd-Tm, have been determined and their crystal structures reported to be isotypic with the Fe 3 C (or Al 3 Ni) type structure which includes also Dy 3 Ni (Lemaire & Paccard, 1967). According to the phase diagram of the Dy-Ni system (Zheng & Wang, 1982), Dy 3 Ni is stable below 1035 K and is formed by the peritectic reaction: Dy + L → Dy 3 Ni.
Similar isotypic RE 3 Co compounds were also reported (Buschow & van der Goot, 1969) for RE = Y, La, Pr, Nd, Sm, Gd-Er. Lu 3 Co has been prepared by Givord & Lemaire (1971), Lu 3 Ni by Romaka et al. (2011). Tsvyashchenko (1986) synthesized Yb 3 Co and Yb 3 Ni at high pressure. According to Tsvyashchenko (1986), Yb 3 Co adopts the Fe 3 C type structure and Yb 3 Ni the Al 3 Ni structure type. On the other hand, Lemaire & Paccard (1967) claimed the RE 3 Ni compounds to have the same structure as the RE 3 Co compounds. To clarify the confusion with the assigned structure types, we have studied literature data for the Fe 3 C (Hendricks, 1930) and the Al 3 Ni (Bradley & Taylor, 1937) prototype structures, concluding that Al 3 Ni and Fe 3 C are isotypic. In accordance with the majority in literature, we will use the Fe 3 C structure type for classification as it has been reported earlier.
Recently, two binary compounds of the Dy-Ni system have been redetermined using single-crystal X-ray data (Levytskyy et al., 2012a,b). Here we present the results of the single-crystal X-ray analysis of Dy 3 Ni. Details of the crystal structure have not been investigated before, and only isotypism with the Fe 3 C was reported together with lattice parameters (Lemaire & Paccard, 1967).
The structure of Dy 3 Ni is characterized by formation of trigonal prisms of Dy atoms with Ni atom enclosed in the centre. A view of the crystal structure of Dy 3 Ni is shown in Fig. 1. The value of the displacement parameter U 22 for the Ni atom displays a high anisotropy in the b direction which may have an influence on some physical properties of the compound. Magnetic properties of Dy 3 Ni were reported by Talik et al. (1996) and generally confirm this assumption which is also valid for the isotypic Dy 3 Co (Baranov et al., 1995).
In Fig. 2  The analysis of interatomic distances shows a slight decrease of some Dy-Ni distances. This feature is in good agreement with the observed Ho-Co distances for previously reported Ho 3 Co (Buschow & van der Goot, 1969). The explanation of this fact may be deduced from an electronic band structure calculation.

Experimental
The sample was prepared from 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) pieces (Aldrich). 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 under an argon atmosphere for four weeks at 870 K.
Shiny metallic gray plate-like 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 Fe 3 C structure type (Hendricks, 1930) and were used as starting point for the structure refinement. The highest Fourier difference peak of 2.82 e Å -3 is at (0.0340 0.75 0.1598) and 1.36 Å away from the Dy2 atom. The deepest hole of -2.65 e Å -3 is at (0.0358 0.25 0.0220) and 1.01 Å away from the Ni atom.

Computing details
Data SHELXL2013 (Sheldrick, 2008) and WinGX (Farrugia, 2012); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010). The ac projection of the unit cell and coordination polyhedra for all types of atoms in the Dy 3 Ni structure.