CUBIC-TO-TETRAGONAL DISPLACIVE TRANSFORMATION IN GD2O3-BI2O3 CERAMICS

Gd2O3-doped Bi2O3 polycrystalline ceramics containing between 2 and 7 mol% Gd2O3 were fabricated by pressureless sintering Powder compacts. The as-sintered samples were tetragonal at room temperature. High-temperature X-ray diffraction (XRD) traces showed that the samples were cubic at elevated temperatures and transformed into the tetragonal polymorph during cooling. On the basis of conductivity measurements as a function of temperature and differential scanning calorimetry (DSC), the cubic --> tetragonal as well as tetragonal --> cubic transition temperatures were determined as a function of Gd2O3 concentration. The cubic --> tetragonal transformation appears to be a displacive transformation. It was observed that additions of ZrO2 as a dopant, which is known to suppress cation interdiffusion in rare-earth oxide-Bi2O3 systems, did not suppress the transition, consistent with it being a displacive transition. Annealing of samples at temperatures less-than-or-equal-to 660-degrees-C for several hundred hours led to decomposition into a mixture of monoclinic and rhombohedral phases. This shows that the tetragonal polymorph is a metastable phase.