Phonons and oxygen diffusion in Bi₂O₃and (Bi_0.7Y_0.3)₂O₃

We report investigation of phonons and oxygen diffusion in Bi₂O₃and (Bi_0.7Y_0.3)₂O₃. The phonon spectra have been measured in Bi₂O₃at high temperatures up to 1083 K using inelastic neutron scattering.Ab initiocalculations have been used to compute the individual contributions of the constituent atoms in Bi₂O₃and (Bi_0.7Y_0.3)₂O₃to the total phonon density of states. Our computed results indicate that as temperature is increased, there is a complete loss of sharp peak structure in the vibrational density of states.Ab initiomolecular dynamics simulations show that even at 1000 K in δ-phase Bi₂O₃, Bi-Bi correlations remain ordered in the crystalline lattice while the correlations between O-O show liquid like disordered behavior. In the case of (Bi_0.7Y_0.3)₂O₃, the O-O correlations broadened at around 500 K indicating that oxygen conductivity is possible at such low temperatures in (Bi_0.7Y_0.3)₂O₃although the conductivity is much less than that observed in the undoped high temperature δ-phase of Bi₂O₃. This result is consistent with the calculated diffusion coefficients of oxygen and observation by quasielastic neutron scattering experiments. Ourab initiomolecular dynamics calculations predict that macroscopic diffusion is attainable in (Bi_0.7Y_0.3)₂O₃at much lower temperatures, which is more suited for technological applications. Our studies elucidate the easy directions of diffusion in δ-Bi₂O₃and (Bi_0.7Y_0.3)₂O₃.