  We report investigation of phonons and oxygen diffusion in Bi2O3 and
(Bi0.7Y0.3)2O3. The phonon spectra have been measured in Bi2O3 at high
temperatures up to 1083 K using inelastic neutron scattering. Ab-initio
calculations have been used to compute the individual contributions of the
constituent atoms in Bi2O3 and (Bi0.7Y0.3)2O3 to 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-initio molecular dynamics simulations show that even at 1000 K in
{\delta}-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 (Bi0.7Y0.3)2O3, the O-O correlations broadened at
around 500 K indicating that oxygen conductivity is possible at such low
temperatures in (Bi0.7Y0.3)2O3 although the conductivity is much less than that
observed in the undoped high temperature {\delta}-phase of Bi2O3. This result
is consistent with the calculated diffusion coefficients of oxygen and
observation by QENS experiments. Our ab-initio molecular dynamics calculations
predict that macroscopic diffusion is attainable in (Bi0.7Y0.3)2O3 at much
lower temperatures, which is more suited for technological applications. Our
studies elucidate the easy directions of diffusion in {\delta}-Bi2O3 and
(Bi0.7Y0.3)2O3.
