Crystal Structures of Sr₄Sn₂Se₉ and Sr₄Sn₂Se₁₀ and the Oxidation State of Tin in an Unusual Geometry

The new ternary selenostannates Sr4Sn2Se9 and Sr4Sn2Se10 have been synthesized by heating the elements at 1023 K in an argon atmosphere. Their structures were determined by single-crystal X-ray methods. Sr4Sn2Se9 crystallizes in a new structure type (Pbam, a = 12.042(2) Å, b = 16.252(3) Å, c = 8.686(2) Å, Z = 4) with Sn2Se64-, SnSe44-, and Se22- subunits. Sr4Sn2Se10 (P21212, a = 12.028(2) Å, b = 16.541(3) Å, c = 8.611(2) Å, Z = 4) has a similar structure with Se32- triangles instead of Se22- dumbbells. Strontium is 8-fold-coordinated by selenium in both cases. The opening angles between tin and the terminal selenium atoms in the Sn2Se6 subunits are close to 160°, which is nearer a typical Sn2+ coordination geometry than classical SnSe4 tetrahedra. This result suggests the tin oxidation state in the Sn2Se6 units to be lower than the expected Sn4+. This question is examined by self-consistent LMTO and LAPW band structure calculations expanded by the Bader analysis of the charge density to assign reliable atomic charges.