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  • Title: Crystal structures of Sr4Sn2Se9 and Sr4Sn2Se10 and the oxidation state of tin in an unusual geometry.
    Author: Pocha R, Johrendt D.
    Journal: Inorg Chem; 2004 Oct 18; 43(21):6830-7. PubMed ID: 15476384.
    Abstract:
    The new ternary selenostannates Sr(4)Sn(2)Se(9) and Sr(4)Sn(2)Se(10) have been synthesized by heating the elements at 1023 K in an argon atmosphere. Their structures were determined by single-crystal X-ray methods. Sr(4)Sn(2)Se(9) crystallizes in a new structure type (Pbam, a = 12.042(2) A, b = 16.252(3) A, c = 8.686(2) A, Z = 4) with Sn(2)Se(6)(4-), SnSe(4)(4-), and Se(2)(2-) subunits. Sr(4)Sn(2)Se(10) (P2(1)2(1)2, a = 12.028(2) A, b = 16.541(3) A, c = 8.611(2) A, Z = 4) has a similar structure with Se(3)(2-) triangles instead of Se(2)(2-) dumbbells. Strontium is 8-fold-coordinated by selenium in both cases. The opening angles between tin and the terminal selenium atoms in the Sn(2)Se(6) subunits are close to 160 degrees , which is nearer a typical Sn(2+) coordination geometry than classical SnSe(4) tetrahedra. This result suggests the tin oxidation state in the Sn(2)Se(6) units to be lower than the expected Sn(4+). 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.
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