Influence of calcination temperature on the structure and optical properties of Bi_3.25La_0.75Ti₃O₁₂ powders

Lanthanum-modified bismuth-titanate powders (BLT) were synthesized using a solid reaction method at various calcination temperatures for 2.5 h under an ambient environment. The evolutions of the structural and the optical properties were analyzed by using X-ray diffraction and ultraviolet visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. The analysis showed that the samples calcined at 700 .deg. C crystallized in an orthorhombic structure with an optimal chemical ratio. The samples calcined at temperature below 500 .deg. C resulted in the α-Bi₂O₃ phase. The structure and the optical bandgap of the sample calcined at 600 .deg. C showed a decrease in the optical bandgap due to the incomplete formation of an orthorhombic BLT structure. The samples calcined at high temperatures exhibited expansion of the unit-cell volume as compared to the optimal samples. In those samples, Bi vacancies due to the high temperature annealing were responsible for changes in the UV PL profile. The relative intensity of the higher energy part (green) in the PL spectra increased with increasing calcination temperature, even though the origin of the shift in the PL spectra might be different at each temperature.