DISTORTIONS OF GALLIUM OCTAHEDRA IN BA5GA6 AND THE LIKELIHOOD OF THE REPORTED PHASE BEING BA5GA6H2

In the intriguing Ba5Ga6 phase of Fornasini and Pani, one finds isolated, nearly perfect octahedral Ga-6 clusters with one more skeletal electron pair (i.e., Ga-6(10-)) than predicted by the Wade-Mingos rules. The electronic structures of the molecular Ga octahedron and Ba5Ga6 were studied by the extended Huckel method. According to our calculations, the octahedral cluster, either as isolated Ga-6(10-) or in the extended Ba5Ga6 structure, appears to be unstable with respect to Jahn-Teller distortion. There is room for hydrides in the Ba5Ga6 structure; Ba5Ga6H2 is an attractive formulation, for it should have, as our computations indicate, relatively undistorted Ga-6 octahedra in it.