Neutron total scattering study of the δ and β phases of Bi ₂O₃

The highly disordered structure of the δ phase of Bi ₂O₃, which possesses the highest known oxide-ion conductivity, has been studied using neutron powder diffraction. A detailed analysis of data collected at 1033(3) K using Rietveld refinement indicates that the time-averaged structure of δ-Bi₂O₃ can be described using the accepted model of a disordered, anion-deficient fluorite structure in space group Fm3m. However, reverse Monte Carlo modelling of the total (Bragg plus diffuse) scattering demonstrates that the local anion environment around the Bi³⁺ resembles the distorted square pyramidal arrangement found within the stable α and metastable β phases at ambient temperature, which is characteristic of the cation's 6s² lone-pair configuration. Similarities between the structures of the highly disordered δ phase and the ambient temperature metastable β phase are used to support this assignment and assess the validity of previous structural models based on short-range ordering of vacancies within the cubic lattice of δ-Bi₂O₃.