A high temperature x-ray diffraction investigation of the β-(Bi₂O₃)_1-x(SrO)_x solid solution

The incidence of phase transitions in the β-(Bi₂O₃)_1-x (SrO)_x solid solution was examined over the composition range x = 0.18-0.42 using in situ high temperature X-ray diffraction. The a lattice parameter of the rhombohedral structure (referenced to hexagonal axes) is approximately constant across the entire composition range of the β-phase. The c parameter also remains approximately constant at compositions of x ≤ 0.28, but at x > 0.28 this parameter shows a marked increase in compositional dependence. The temperature dependence of the a and c lattice parameters exhibits discrete discontinuities at distinct temperatures T₀ which occur between 600 and 710 °C depending on the composition. These fully reversible effects confirm previous reports of structural discontinuities, and are consistent with observations of enhanced ionic conductivity, differential thermal analysis (DTA) peaks and transmission electron microscope (TEM) superlattice reflections indicative of an order:disorder transition in the β-phase. The maximum magnitude of the associated discontinuity in the volume of the unit cell occurs at the stoichiometric composition of x = 0.28 which represents an equal number of Bi³⁺ and Sr²⁺ ions. The c parameter/temperature plot for the composition x = 0.42 shows a distinct increase in slope at a temperature T₁ which is close to 490 °C. This effect is attributed to the relief of localized microdomains of order or the progressive disorder of a superlattice with a considerable defect structure. The transition temperatures T₀ and T₁ correlate closely with previously reported temperatures of discrete increases in oxygen ion conductivity and of DTA peaks and enable an effective composition/temperature boundary to be defined between the disordered and ordered β-phase structures.