Unique barium selenostannate-selenide:: Ba7Sn3Se13 (and its variants Ba7Sn3Se13-δTeδ) with SnSe4 tetrahedra and isolated Se anions

The new selenostannate-selenide Ba7Sn3Se13 and its quaternary substitution variants Ba7Sn3Se13-delta Te-6 were prepared from the elements at 900 degrees C followed by slow cooling to 200 degrees C. Ba7Sn3Se13 crystallizes in the orthorhombic space group Pnma, with lattice parameters of alpha = 12.748(1) angstrom, b = 24.891(3) angstrom, c = 9.212(1) angstrom, and V = 2923.0(5) angstrom(3) (Z = 4). The structure motifs include Ba2+ cations, isolated SnSe4 tetrahedra, and isolated Se2- anions that are not bonded to any Sn atom. Partial Se/Te replacements are possible up to 2.24(2) of the 13 Se atoms, and the Te atoms prefer the naked site, surrounded by six Ba2+ cations, as well as the site with the longest Se-Ba distances. The Se/Te substitution leads to a smaller band gap as computed with the LMTO approximation, which becomes visible by the gradual darkening of the red color with increasing Te content.