Redetermination of LaZn5 based on single crystal X-ray diffraction data

The crystal structure of the already known binary title compound LaZn5 (lanthanum pentazinc) (space group P6/mmm, Pearson symbol hP6, CaCu5 structure type) has been redetermined from single-crystal X-ray diffraction data. In contrast to previous determinations based on X-ray powder data [Nowotny (1942). Z. Metallkd. 34, 247–253; de Negri et al. (2008). Intermetallics, 16, 168–178], where unit-cell parameters and assignment of the structure type were reported, the present study reveals anisotropic displacement parameters for all atoms. The crystal structure consists of three crytallographically distinct atoms. The La atom (Wyckoff site 1a, site symmetry 6/mmm) is surrounded by 18 Zn atoms and two La atoms. The coordination polyhedron around one of the Zn atoms (Wyckoff site 2c, site symmetry -6m2) is an icosahedron made up from three La and nine Zn atoms. The other Zn atom (Wyckoff site 3g, site symmetry mmm) is surrounded by four La and eight Zn atoms. Bonding between atoms is explored by means of the TB–LMTO–ASA (tight-binding linear muffin-tin orbital atomic spheres approximation) program package. The positive charge density is localized around La atoms, and the negative charge density is around Zn atoms, with weak covalent bonding between the latter.


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Financial support from the Ministry of Education and Science, Youth and Sport of Ukraine (No. 0111U001089) is gratefully acknowledged.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2565).

References
Andersen, K., Povlovska, Z. & Jepsen, O. (1986). Phys. Rev. B, 34, 51-53. Berche, A., Record, M.-C. & Rogez, J. (2009). Open Thermodynam. J. 3, 7-16. Brandenburg, K. (2006. DIAMOND. Crystal Impact GbR, Bonn, Germany. Bruker (2004  respect to the structure of the compound LaZn 4 . In order to determine its crystal structure a sample with the same composition was synthesized. However, this sample was prepared under non-equilibrium conditions and phase analysis from X-ray powder data revealed the presence of LaZn 5 , LaZn 2 , trace amounts of LaZn and strong reflections of unknown phase(s). The lattice parameters of the title LaZn 5 phase were determined for the first time based on X-ray powder diffraction data (Nowotny, 1942). Previous authors (Nowotny, 1942;de Negri et al., 2008) also assigned the structure type. However, a complete crystal structure determination including anisotropic displacement parameters was not carried out before. Therefore a highquality single-crystal of LaZn 5 was selected and the results of the full structure determination are presented in this paper.
The crystal structure consists of three crytallographically distinct atoms. La1 (Wyckoff site 1a, site symmetry 6/mmm) is surrounded by 18 Zn atoms and 2 La atoms. The coordination polyhedron around Zn1 atom (Wyckoff site 2c, site symmetry -6m2) is an icosahedron formed by 3 La and 9 Zn atoms. Zn2 (Wyckoff site 3g, site symmetry mmm) is surrounded by 4 La and 8 Zn atoms. The projection of the LaZn 5 unit cell is given in Fig. 1. The thermal displacement of the lanthanum atom is almost isotropic. The thermal ellipsoids of the Zn1 atoms are oblate along the c axis. The thermal ellipsoids of the Zn2 atoms are extended along the a and b axes due to the largest space for displacement in this direction (the distances of corresponding atoms to each other and to La atoms are larger than for Zn1 atoms).
The electronic structure of LaZn 5 was calculated using the TB-LMTO-ASA package (Andersen et al., 1986). The dominant type of bonding in this compound is metallic. The La atoms donate their electrons to the Zn atoms. Therefore positive charge density can be observed around the rare earth atom and negative charge density is around the transition metal atoms.
This fact, together with significant electron density (~0.4 e/Å 3 ) and significant ELF density (~0.4) between Zn atoms, confirms the weak covalent bonding between them. In other words, an ion-metallic bonding between La and Zn atoms and a covalent-metallic bonding between Zn atoms is evident (Figure 2). A similar way of bond formation is also observed for pressing into a pellet. This pellet was sealed in an evacuated silica ampoule and annealed in a resistance furnace at 873 K for 30 days and subsequently quenched in cold water. No reaction between the alloy and the silica container was observed.

Refinement
The highest peak of 1.11 e/Å 3 is at (0; 0; 0.1905) and 0.81 Å away from the La1 atom. The deepest hole -0.71 e/Å 3 is at (0; 0; 1/2) and 2.13 Å away from the same atom. Fig. 1. Projection of the unit cell, coordination polyhedra of the atoms and the covalent bonds in the LaZn 5 compound. Atoms are given with their anisotropic displacement ellipsoids at the 99.99% probability level.  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.