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 Ba/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 Cmc2₁ with a = 9.3981(3) Å, b = 25.7192(7) Å, c = 8.7748 (3) Å, and Z = 4. A rational synthesis has been devised at 600°C. 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]^-_∞ backbone with [SiSe₄]^4- tetrahedra chelating every other Sb atom from the same side of the backbone. The V-shaped triselenide groups, (Se₃)^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°C.