Syntheses, structures, and optical properties of Ba 4 Ga 4 SnSe 12 and Ba 6 Ga 2 SnSe 11

Two new quaternary selenides, namely Ba 4 Ga 4 SnSe 12 and Ba 6 Ga 2 SnSe 11 , 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. Ba 4 Ga 4 SnSe 12 crystallizes in the non-centrosymmetric space group P 4̄2 1 / c of the tetragonal system and has a three-dimensional structure. Its three-dimensional framework is built up from corner-sharing GaSe 4 and SnSe 4 tetrahedra with eight-coordinated Ba 2+ cations residing in the cavities. Ba 6 Ga 2 SnSe 11 crystallizes in a new structure type in the monoclinic centrosymmetric space group P 2 1 / c . The structure of Ba 6 Ga 2 SnSe 11 features a zero-dimensional structure containing totally isolated distorted SnSe 4 tetrahedra and a discrete Ga 2 Se 7 unit with Ba 2+ 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 Ba 4 Ga 4 SnSe 12 and Ba 6 Ga 2 SnSe 11 respectively. In addition, the electronic structure calculation of Ba 4 Ga 4 SnSe 12 indicates that it is a direct-gap semiconductor with the band gap mainly determined by the [Ga 4 SnSe 12 ] 8− anionic framework.