Syntheses, structures, and optical properties of Ba4Ga4SnSe12 and Ba6Ga2SnSe11

Abstract

Two new quaternary selenides, namely Ba₄Ga₄SnSe₁₂ and Ba₆Ga₂SnSe₁₁, have been synthesized for the first time, representing the first two members in the A/M/Sn/Q (A = alkaline-earth metal; M = Al, Ga, In; Q = S, Se, Te) system. Ba₄Ga₄SnSe₁₂ crystallizes in the non-centrosymmetric space group P2₁/c of the tetragonal system and has a three-dimensional structure. Its three-dimensional framework is built up from corner-sharing GaSe₄ and SnSe₄ tetrahedra with eight-coordinated Ba²⁺ cations residing in the cavities. Ba₆Ga₂SnSe₁₁ crystallizes in a new structure type in the monoclinic centrosymmetric space group P2₁/c. The structure of Ba₆Ga₂SnSe₁₁ features a zero-dimensional structure containing totally isolated distorted SnSe₄ tetrahedra and a discrete Ga₂Se₇ unit with Ba²⁺ cations located between them. On the basis of the diffuse-reflectance spectra, the band gaps are 2.16 (2) eV and 1.99 (2) eV for Ba₄Ga₄SnSe₁₂ and Ba₆Ga₂SnSe₁₁ respectively. In addition, the electronic structure calculation of Ba₄Ga₄SnSe₁₂ indicates that it is a direct-gap semiconductor with the band gap mainly determined by the [Ga₄SnSe₁₂]⁸⁻ anionic framework.