Zr3NiSb7: a new antimony-enriched ZrSb2 derivative

Single crystals of trizirconium nickel heptaantimonide were synthesized from the constituent elements by arc-melting. The compound crystallizes in a unique structure type and belongs to the family of two-layer structures. All crystallographically unique atoms (3 × Zr, 1 × Ni and 7 × Sb) are located at sites with m symmetry. The structure contains ‘Zr2Ni2Sb5’ and ‘Zr4Sb9’ fragments and might be described as a new ZrSb2 derivative with a high Sb content.

with MgAgAs and Y 3 Au 3 Sb 4 type structures. The investigation of new intermetallic phases is useful for the development of new materials, and the accurate determination of their crystal structures is a basic requirement for a better understanding of the corresponding physical properties.
Investigation of the Zr-Ni-Sb ternary system revealed the presence of several compounds in the Sb-enriched area (Romaka et al., 2007), including the new title antimonide with composition Zr 27 Ni 9 Sb 64 (in % at ). Interatomic distances (Table   1) between Sb atoms are in good agreement with the sum of the atomic radius (Emsley, 1991), whereas the majority of Zr-Sb and all Ni-Sb distances are somewhat shortened. Such shortening may be explained by partial covalent bonding which appears to be significant between Ni-Sb atoms because their contact distances are rather close to the sum of their covalent radii (2.56 Å). As the majority of ternary intermetallics are constructed from the fragments of their most stable binary compounds, the structure analysis of the antimonides Zr 3 NiSb 7 and the already known Zr 2 NiSb 3 (Tkachuk et al., 2007) in the Sb-enriched area shows that both can be derived from the binary compound ZrSb 2 (Garcia & Corbett, 1988), which crystallizes in the PbCl 2 structure type.
Zr 3 NiSb 7 belongs to the family of two-layer structures. It may be represented as a net of trigonal prisms formed by Sb atoms that are bridged by nickel atoms (Fig. 1a). Such an arrangement is very similar to that in the binary ZrSb 2 structure ( Fig. 1b). The coordination polyhedra are distorted tri-capped trigonal prisms for the Zr atoms, and distorted octahedra for Ni atoms. In an alternative description, the Zr 3 NiSb 7 structure contains fragments of the hypothetical "Zr 2 Ni 2 Sb 5 " and "Zr 4 Sb 9 " structures ( Fig. 2) which are so far unknown for the ternary Zr-Ni-Sb or binary Zr-Sb systems. The main feature of the Zr 3 NiSb 7 structure is the absence of covalent bonding between antimony atoms in contrast to the ZrSb 2 structure. The general conclusion is that the presence of Ni atoms intensifies the interaction between Zr/Ni and Sb and, at the same time, reduces the bonding between Sb atoms. One may speculate that the composition of the Zr 3 NiSb 7 compound may be the boundary limit of some solid solutions based on ZrSb 2 . However, the detailed study of the phase equilibria in the Zr-Ni-Sb system did not show a formation of any substitutional or interstitial solid solution. Moreover, the diffraction patterns of Zr 3 NiSb 7 and ZrSb 2 are rather different.

Experimental
A sample with nominal composition Zr 30 Ni 10 Sb 60 was prepared by arc-melting the constituent elements Zr (99.99 wt.%), Ni (99.99 wt.%), and Sb (99.99 wt.%) on a water-cooled copper hearth under a protective Ti-gettered argon atmosphere. 5 wt.% excess of Sb was required to compensate the evaporative loss during arc-melting. The ingot was annealed at 870 K for 720 h in an evacuated silica ampoule, and finally quenched in cold water. A crystal of the title compound suitable for single-crystal X-ray diffraction was extracted directly from the annealed sample. The chemical composition of the crystal was determined on the basis of an energy dispersive X-ray spectroscopical analysis using a Hitachi S-2700 scanning electron microscope. The result of the analysis is in good aggreement with the composition calculated from the structural refinement: Measured: 24.5 (8) % at Zr, 11.3 (6) % at Ni, 64.2 (16) % at Sb; calculated Zr 27 % at , Ni 9% at , Sb 64 % at .

Refinement
The highest remaining electron density peak and the deepest hole are located 0.80 Å from Sb1 and 1.78 Å from Ni1, respectively. The structure solution and refinement were also performed in the non-centrosymmetric space group Pna2 1 , but were less satisfactory and resulted in larger R indices and atomic displacement parameters. Fig. 1. (a). Projection of the Zr 3 NiSb 7 structure onto the (010) plane with displacement ellipsoids drawn at the 95% probability level. [Symmetry codes: (i) 0.5 -x, 1 -y, -1/2 + z; (iv) 0.5x, -y, 0.5 -z; (vi) 1/2 + x, y, 1.5 -z]; (b) Projection of the ZrSb 2 structure onto the (010) plane. Fig. 2. The stacked "Zr 2 Ni 2 Sb 5 " and "Zr 4 Sb 9 " fragments in the Zr 3 NiSb 7 structure.  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 Rfactors(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.