Ba5Ga6: a phase with octahedral clusters of gallium

The crystal structure of Ba5Ga6 was determined by single-crystal diffractometry. It is hexagonal, hP22, space group P6c2 (n. 188), a = 7.771(2) A ̊, c = 14.376(4) A ̊, Z = 2, refined with anisotropic displacement factors, R = 0.076 with 358 reflections, and contains a disordered Ba position with partial occupation. The structure is characterized by octahedral Ga6 clusters, isolated and surrounded by a cuboctahedron of twelve Ba atoms. The barium cuboctahedra, by sharing of edges and faces, are condensed in a tridimensional framework. Application of Wade's rules to the Ga6 naked cluster gives an excess of two electrons per formula unit. The compound Ba5Ga6 replaces the phase "BaGa" given in the original Ba-Ga phase diagram.