La5Zn2Sn

A single crystal of pentalanthanum dizinc stannide, La5Zn2Sn, was obtained from the elements in a resistance furnace. It belongs to the Mo5SiB2 structure type, which is a ternary ordered variant of the Cr5B3 structure type. The space is filled by bicapped tetragonal antiprisms from lanthanum atoms around tin atoms sharing their vertices. Zinc atoms fill voids between these bicapped tetragonal antiprisms. All four atoms in the asymmetric unit reside on special positions with the following site symmetries: La1 (..m); La2 (4/m..); Zn (m.2m); Sn (422).

A single crystal of pentalanthanum dizinc stannide, La 5 Zn 2 Sn, was obtained from the elements in a resistance furnace. It belongs to the Mo 5 SiB 2 structure type, which is a ternary ordered variant of the Cr 5 B 3 structure type. The space is filled by bicapped tetragonal antiprisms from lanthanum atoms around tin atoms sharing their vertices. Zinc atoms fill voids between these bicapped tetragonal antiprisms. All four atoms in the asymmetric unit reside on special positions with the following site symmetries: La1 (..m); La2 (4/m..); Zn (m.2m); Sn (422).
La2 atoms are enclosed into trigon -tetrahexahedron with CN=14. And coordination polyhedra of the Zn atoms are bicapped trigonal prisms with CN=9. Coordination polyhedra of Sn atoms share their vertices forming three dimensional framework.
The voids in this framework are filled by zinc atoms. (See graphical abstract).
The way of bond formation in this compound was assumed using only X-ray diffraction data. Further structure refinement was carried out by means of Jana2006 software package using anharmonic ADP for La1 and Zn atoms. Anharmonic displacement parameters for other atoms were not refined because in case of their refinement their standard deviations were larger than obtained values. As the result we gained lower absolute values of peak and hole in the difference Fourier map (1.02 and -1.17 e Å -3 respectively). The resulting isosurface drawn at the level 0.308 e/Å 3 and sections of difference Fourier map are given in Fig. 2. These maps and sections are noisy but some trends in location of positive and negative regions can be noticed. Positive residual electron density is mostly situated around zinc atoms and near layers made of tin atoms. Negative residual density is mostly located between lanthanum atoms which means that lanthanum atoms donate their electrons to zinc and tin atoms. Similar behaviour of lanthanum atoms can be observed in the LaZn 12.37 compound using electronic structure calculations (See Oshchapovsky et al., (2011)). As a conclusion this compound besides dominate metallic bonding has a weak ionic interaction between lanthanum and zinc and tin atoms.

Experimental
Small good quality single-crystal of title compound was isolated from alloy with composition La 7 ZnSn 2 during systematic investigation of lanthanum-rich region of La-Zn-Sn ternary system. The samples with high lanthanum contents were prepared by melting of pieces of pure metals in evacuated quartz ampoule with subsequent annealing at 600 0 C for 30 days.
Further phase analysis showed the existence of title compound in sample with composition La 7 ZnSn 2 as well as in the other lanthanum-rich ternary alloys. However they were non equilibrium. Fig. 1