Phase transition in ferroelectric SrBi₂Ta₂O₉ thin films with change of heat-treatment temperature

Changes in the composition and the crystal structure of micro-grains have been studied using transmission electron microscopy (TEM) and energy dispersive X-ray microanalysis in order to clarify the phase formation of ferroelectric SrBi₂Ta₂O₉ (SBT) with heat-treatment at 650-750°C. Micrograins crystallized at 650°C begin to change to bismuth-layer-structure-family (BLSF) grains at temperatures exceeding 700°C, and form pure-BLSF phase grains at 800°C. Hence, a micrograins phase exists in thin films at 650-750°C and we determined that the bismuth composition in these grains decreases. Using selected diffraction and high-resolution TEM, we found that a pyrochlore phase was formed in the bismuth-poor micrograins that were heat treated at 750°C. The formation of pyrochlore made pure-phase BLSF difficult to obtain.