Structure and oxide ion conductivity in Bi28Re2O49, a new bismuth rhenium oxide containing tetrahedral and octahedral Re(VII)

A new Bi-Re-O phase, Bi28Re2O49, has been synthesized and characterized. Its structure, determined by neutron powder diffraction, is a superstructure of the cubic fluorite unit cell: tetragonal, I4/m, a = 8.7216(1) Angstrom, c = 17.4177(2) Angstrom. The structure comprises an ordered framework of linked BiO(4)e trigonal bipyramids and square pyramids (e = lone pair of electrons), with discrete Re oxoanions at the origin and body-centre of the unit cell. The infra-red spectrum and Re K-edge X-ray absorption spectrum were consistent with the presence of tetrahedral ReO4- and octahedral ReO65- species in the ratio of 3 : 1. This correctly provides charge balance in the overall structure. The phase is found to display high oxide ion conductivity, which may relate to the presence of the two different oxoanion species, and possible migration of O-2(-) ions between them.