Si extraction from silica in a basic polychalcogenide flux.: Stabilization of Ba4SiSb2Se11, a novel mixed selenosilicate/selenoantimonate with a polar structure

An unusual compound, Ba4SiSb2Se11, was discovered from a reaction of Br/Th/Sb/Se. It is assumed that Si was extracted from the silica reaction tube. It forms as silver needlelike crystals in the polar space group Cmc2(1) with a = 9.3981(3) Angstrom, b = 25.7292(7) Angstrom, c = 8.7748 (3) Angstrom. and Z = 1. A rational synthesis has been devised at 600 OC. The compound is composed of Ba2+ ions stabilized between infinite one-dimensional [SiSb2Se11](8-) chains running parallel to the a axis. Each chain is composed of a [SbSe2](infinity)(-) backbone with [SiSe4](4-) tetrahedra chelating every other Sb atom from the same side of the backbone. The V-shaped triselenide groups, (Se-3)(2-), are attached to the rest of the Sb atoms in the chain through one of their terminal Se atoms. The compound has a band gap of 1.43 eV. The Raman spectrum shows a broad shift at 247 cm(-1) and a shoulder around 234 cm(-1), which are related to the Se-Se vibration of the triselenide groups and/or the Si-Se vibrations of the [SiSe4](4-) groups. The compound decomposes at 522 degreesC.