TEMPERATURE-DEPENDENCE OF THE TI-SITE ELECTRIC-FIELD GRADIENT IN TITANITE, CATISIO5

Using perturbed-angular-correlation (PAC) spectroscopy, we have measured nuclear-electric-quadrupole interactions at the Ti sites in ceramic samples of titanite that were doped with the Hf-181 --> Ta-181 PAC probe. Measurements were made over a temperature range from 10 to 980 K to investigate the effects of the antiferroelectric-to-paraelectric transition at approximately 500 K on the electric-field gradients (EFG's) at the Ti site. Near the transition temperature T(c), the magnitude V(zz) of the EFG shows no significant inflections. Over the entire temperature range, V(zz) decreases approximately linearly with increasing temperature, whereas eta remains relatively constant. The line-shape parameter delta decreases slowly with increasing temperature from 10 K to approximately T(c); at temperatures above T(c), delta remains relatively constant. Using a point-charge model, we attribute the absence of any discernible effects of the transition on the measured values of V(zz) and eta to the direction and symmetry of the Ti-ion displacement relative to the Ti-site EFG axes.