PERTURBED-ANGULAR-CORRELATION SPECTROSCOPY - EVIDENCE FOR PROBE DOPANT INTERACTIONS IN CA-DOPED BARIUM-TITANATE

To investigate dopant- and defect-probe interactions, we have performed perturbed-angular-correlation (PAC) spectroscopy on Ca-doped barium titanate ceramics. Polycrystalline samples of barium titanate were prepared using the resin-intermediate process, and the samples were doped with Ca according to the formula (Ba1-xCax)(Ti1-yCay)O3-y, where 0 less-than-or-equal-to x less-than-or-equal-to 0.05 and 0 less-than-or-equal-to y less-than-or-equal-to 0.05. All of the samples were doped with approximately 0.1 at.% Hf, which was substituted into the Ti sites and carried the Hf-181-->Ta-181 PAC probe radioactivity. Measurements were performed over a temperature range from laboratory temperature to approximately 1100 K. In the ferroelectric phases, at Ca concentrations of several percent, the doping produces an increase in the Ti-site electric-field-gradient (efg) asymmetry and in the extent of spectral linebroadening. At higher Ca concentrations, the linebroadening increases further and the ferroelectric-to-paraelectric transition is not discernable. The changes in efg symmetry and spectral linebroadening, which are essentially temperature invariant, could be attributed to preferential substitution of the Ca dopant into Ba coordination spheres that are near the probe sites. In the paraelectric phase, doping at 2 at.% produces a weak perturbation that is several times stronger than the corresponding weak perturbation that non-doped samples show over the same temperature range. This result strongly suggests that O vacancies do not cause these weak perturbations.