P. Su et al., CUBIC-TO-TETRAGONAL DISPLACIVE TRANSFORMATION IN GD2O3-BI2O3 CERAMICS, Journal of the American Ceramic Society, 76(10), 1993, pp. 2513-2520
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-temp
erature X-ray diffraction (XRD) traces showed that the samples were cu
bic at elevated temperatures and transformed into the tetragonal polym
orph during cooling. On the basis of conductivity measurements as a fu
nction of temperature and differential scanning calorimetry (DSC), the
cubic --> tetragonal as well as tetragonal --> cubic transition tempe
ratures were determined as a function of Gd2O3 concentration. The cubi
c --> tetragonal transformation appears to be a displacive transformat
ion. It was observed that additions of ZrO2 as a dopant, which is know
n to suppress cation interdiffusion in rare-earth oxide-Bi2O3 systems,
did not suppress the transition, consistent with it being a displaciv
e 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 te
tragonal polymorph is a metastable phase.