ELECTRONIC-STRUCTURE AND CHEMICAL BONDING IN DELTA-BI2O3

The cubic fluorite structure oxide, delta-Bi2O3, possesses a very high oxygen ionic conductivity which can be very useful in applications. S ince the structural arrangement of the oxygen vacancies in Bi2O3 canno t be unambiguously established experimentally, we have used the LMTO m ethod in both the full-potential and ASA versions to study the delta-B i2O3 system with various oxygen vacancy configurations. Our results in clude band structures, densities of states, cohesive energies and part ial pressures. We find that there are nonlinear changes in the cohesiv e energy with variations in the oxygen vacancy concentration, with a s tructure with two vacancies per unit cell being the most stable. Our t otal energy calculations lead to the conclusion that the most probable stable configuration is the [111] type of vacancy ordering for Bi2O4, corresponding to equal numbers of vacancies in all of the Bi-O planes , and formation of channels for oxygen diffusion in the crystals. The role of Bi-O and Bi-Bi bonding in the stabilization of such structures is also discussed.