ELECTRONIC FACTORS CONTROLLING BANDWIDTHS IN OXIDES WITH THE PEROVSKITE AND CADMIUM HALIDE STRUCTURES

The electronic structure of oxides with the perovskite and cadmium hal ide structures are derived using tight-binding theory and the angular overlap model. The control of the form of the energy bands by the pres ence of vertex- (perovskite) and edge-sharing (cadmium halide) of the coordination octahedra is first explored using one-dimensional example s. For one-, two-, and three-dimensional structures of the perovskite type, a very interesting result is uncovered. It is shown that on the band model a band gap may only arise if the oxygen 2s orbital is inclu ded in the orbital problem. Prediction of the magnitude of the band ga ps in d(6) perovskites is thus not a simple matter. This band structur e, derived from tight-binding theory via the construction of Bloch orb itals, is compared with an energy band scheme based on local crystal f ield orbitals and covalent metal-oxygen interactions, which has tradit ionally been used in this area.