A high temperature X-ray diffraction investigation of the β-(Bi2O3)1-x(SrO)x solid solution

The incidence of phase transitions in the beta-(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 beta-phase. The c parameter also remains approximately constant at compositions of x less than or equal to 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 T-0 which occur between 600 and 710 degreesC 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 beta-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 T-1 which is close to 490 degreesC. 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 T-0 and T-1 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 beta-phase structures.