ORDER-DISORDER EFFECTS IN THE PHASE-TRANSITIONS OF LINBO3 AND LITAO3 MEASURED BY PERTURBED-ANGULAR-CORRELATION SPECTROSCOPY

Perturbed-angular-correlation (PAC) spectroscopy was used to measure nuclear-electric-quadrupole interactions at the LI sites in two isostructural, ferroelectric ternary-metal oxides, LiNbO3 and LiTaO3. These compounds were prepared as ceramics doped with approximately 0.01 at. % Hf that carried the radioactive Hf-181 --> Ta-181 PAC probes. PAC measurements were made over a temperature range from 295 to almost-equal-to 1100 K, which included the ferroelectric-to-paraelectric transition for LiTaO3. Because the transition temperature T(c) for LiNbO3 exceeded the accessible temperature range of the available apparatus, the investigation focused mainly on the features of the LiTaO3 transition. In particular, the measured perturbation functions show well-defined, high-frequency, static interactions that are characterized by extensive line broadening at temperatures well below T(c) and by significantly less line broadening at temperatures above T(c). At temperatures above T(c), the electric-field-gradient (efg) asymmetry parameter-eta is close to zero, but at temperatures well below T(c), eta is significantly larger than zero. This result is not expected, because the axial symmetry at the Li site associated with the diffraction-derived structure implies that eta should vanish at temperatures both below and above T(c). The observed eta-temperature dependence is explained using an order-disorder model. This model suggests that some of the Li ions (and to some extent group-V antisite defects) occupy normally vacant metal sites and break the axial symmetry associated with the Li site. At temperatures below T(c), the efg component V(zz) increases as temperature increases, and over the same temperature range, the spontaneous polarization decreases. For this reason, V(zz) may not be strongly coupled to the order parameter for the transition. However, the anomalous temperature dependence of eta suggests that eta may be coupled to the order parameter.