Signature of lattice defects in far infrared reflectivity of Ba(Mg_1/3Ta_2/3)O₃

Far infrared reflectivity spectra of Ba(Mg_1/3,Ta_2/3)O₃ prepared at several sintering temperatures were measured, and the eigenfrequencies and damping constants of the TO modes were determined. The reflectivity spectra were fitted with the four-parameter semi-quantum model. The variation in the E_u(O_II) at 222 cm^-1 and A_2u(O_II) at 238 cm^-1 modes in well ordered ceramics was attributed to the variation in the concentration of the B site defects. It was also found that the change in the oxygen partial pressure of the sintering atmosphere causes a change in the seventh (316 cm^-1) and eighth (352 cm^-1) modes. We attribute these changes to the oxygen site defect although we cannot evaluate the concentration of this defect at this moment. From the behavior of the damping constants it is suggested that the Ba(Mg_1/3Ta_2/3)O₃ (BMT) attains equilibrium defect density at the heat treatment temperature of more than 1630 °C (120 h).