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

Two new quaternary selenides, namely Ba sub(4)Ga sub(4)SnSe sub( 12) and Ba sub(6)Ga sub(2)SnSe sub( 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 sub(4)Ga sub(4)SnSe sub( 12) crystallizes in the non-centrosymmetric space group P4&cmb.macr; 2 sub(1)/cof the tetragonal system and has a three-dimensional structure. Its three-dimensional framework is built up from corner-sharing GaSe sub(4) and SnSe sub(4) tetrahedra with eight-coordinated Ba super(2+) cations residing in the cavities. Ba sub(6)Ga sub(2)SnSe sub( 11) crystallizes in a new structure type in the monoclinic centrosymmetric space group P2 sub(1)/c. The structure of Ba sub(6)Ga sub(2)SnSe sub( 11) features a zero-dimensional structure containing totally isolated distorted SnSe sub(4) tetrahedra and a discrete Ga sub(2)Se sub(7) unit with Ba super(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 sub(4)Ga sub(4)SnSe sub( 12) and Ba sub(6)Ga sub(2)SnSe sub( 11) respectively. In addition, the electronic structure calculation of Ba sub(4)Ga sub(4)SnSe sub( 12) indicates that it is a direct-gap semiconductor with the band gap mainly determined by the [Ga sub(4)SnSe sub(12)] super(8-) anionic framework.