Terbium (lithium zinc) distannide, TbLi1–xZnxSn2 (x = 0.2)

The new terbium (lithium zinc) distannide, TbLi1–xZnxSn2 (x = 0.2) crystallizes in the orthorhombic CeNiSi2 structure type with space group Cmcm and Pearson symbol oS16. Of the four independent 4c atom positions (m2m site symmetry), three are fully occupied by individual atoms (two by Sn and one by Tb atoms) and the fourth is occupied by Li and Zn atoms with a statistical distribution. The Tb coordination polyhedron is a 21-vertex pseudo-Frank–Kasper polyhedron. One Sn atom is enclosed in a tricapped trigonal prism, the second Sn atom is in a cuboctahedron and the statistically distributed (Li,Zn) site is in a tetragonal antiprism with one added atom. Electronic structure calculations were used for the elucidation of reasons for and the ability of mutual substitution of lithium and transition metals. Positive charge density was observed around the rare earth atom and the Li and Zn atoms, the negative charge density in the proximity of the Sn atoms.

The new terbium (lithium zinc) distannide, TbLi 1-x Zn x Sn 2 (x = 0.2) crystallizes in the orthorhombic CeNiSi 2 structure type with space group Cmcm and Pearson symbol oS16. Of the four independent 4c atom positions (m2m site symmetry), three are fully occupied by individual atoms (two by Sn and one by Tb atoms) and the fourth is occupied by Li and Zn atoms with a statistical distribution. The Tb coordination polyhedron is a 21-vertex pseudo-Frank-Kasper polyhedron. One Sn atom is enclosed in a tricapped trigonal prism, the second Sn atom is in a cuboctahedron and the statistically distributed (Li,Zn) site is in a tetragonal antiprism with one added atom. Electronic structure calculations were used for the elucidation of reasons for and the ability of mutual substitution of lithium and transition metals. Positive charge density was observed around the rare earth atom and the Li and Zn atoms, the negative charge density in the proximity of the Sn atoms.
TbZnSn 2 with tetragonal symmetry (space group P4/nmm, HfCuSi 2 structure type). Structural studies of the four-component alloys from TbLiSn 2 -TbZnSn 2 sections indicate the existence of TbLi 1-x Zn x Sn 2 (x = 0 -1/5) limited solid solution. In the ternary TbLiSn 2 compound lithium atoms occupy the same crystallographic position that the atoms of transition metal in the original CeNiSi 2 structure type. The same was observed previously when we studied RELiGe with the ZrNiAl type (Pavlyuk et al., 1991 andPavlyuk et al., 1992a) and RE 3 Li 2 Ge 3 with Hf 3 Ni 2 Si 3 type (Pavlyuk & Bodak, 1992b). X-ray single-crystal study showed that the TbLi 1-x Zn x Sn 2 solid solution formed by the partial substitution of lithium atoms by zinc atoms in 4c site. The ability of lithium atoms to substitute the atoms of transition metals we observed previously studying solid solutions RELi x Cu 2-x Si 2 and RELi x Cu 2-x Ge 2 (Pavlyuk et al., 1993). Among the rare earth metals was taken yttrium, which has less number of electrons than majority of other rare earth metals.
According to the results of calculations in the both models the rare earth atoms donate their electrons to tin atoms. The lithium atoms (Fig. 2a) and zinc atoms (Fig. 2 b) also loses their electrons. So positive charge density in various scale can be observed around rare earth, lithium and zinc atoms and negative charge density is around tin atoms. Taking into account these data and also the closeness of the effective radius of zinc and lithium atoms in intermetallic compounds it can be concluded that nothing prevents their mutual substitution.

Experimental
Terbium, lithium, zinc and tin, all with a nominal purity more than 99.9 wt. %, were used as starting elements. First, the pieces of the pure metals with a stoichiometry Tb 25 Li 20 Zn 5 Sn 50 were pressed into pellets, enclosed in a tantalum crucible supplementary materials sup-2 and placed in a resistance furnace with a thermocouple controller. The sample was heated to 670 K at a rate of 5 K/min, maintained over a period of 48 h and then temperature was increased to 1070 K over a period of 10 h. The alloy was annealed at 670 K for 120 h and cooled slowly to room temperature. Small, good-quality single-crystals of the title compound were isolated from an alloy by mechanical fragmentation.

Refinement
The Li position (Wyckoff sites 4c) showed displacement parameters considerably smaller than it should be for the lithium, suggesting that this position is partially occupied by the heavier Zn atom. The refinement of the occupancy of this statistically mixed position showed, that it contains 80% of Li and 20% of Zn atoms. Fig. 1. Unit cell projection and coordination polyhedra of atoms in the TbLi 1-x Zn x Sn 2 compound. Thermal ellipsoids are drawn at a 95% probability level.

Special details
Geometry. All e.s. 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.