Ba5Ga4Se10: a new selenidogallate containing the novel [Ga4Se10](10-) anionic cluster with Ga in a mixed-valence state

The new compound Ba5Ga4Se10 has been synthesized for the first time. It crystallizes in the tetragonal space group I4/mcm with a = 8.752(2) angstrom, c = 13.971(9) angstrom, and Z = 2. The structure contains discrete [Ga4Se10](10-) anions and charge-compensating Ba2+ cations. The novel highly anionic [Ga4Se10](10-) cluster is composed of two Ga(Se)(4) tetrahedra and two Ga(Ga)(Se)(3) tetrahedra with Ga in the 2+/3+ valence states. It also exhibits an unusually long Ga-Se distance of 2.705(2) angstrom, which has only been observed under high pressure conditions before. A band gap of 2.20(2) eV was deduced from the UV/vis diffuse reflectance spectrum.