HIGH-TEMPERATURE PARTITIONING OF HF-181-PROBE IMPURITIES BETWEEN LI AND GROUP-V SITES IN LINBO3 AND LITAO3

Perturbed-angular-correlation (PAC) spectroscopy was used to measure nuclear-electric-quadrupole interactions at high temperatures in ceramic ternary metal oxides, LiNbO3 and LiTaO3. In these ferroelectric ceramics, the Hf-181 --> Ta-181 PAC probe was carried by approximately 0.03 at. % Hf as an impurity dopant, and the PAC measurements were made over a temperature range from approximately 1300 to 1700 K, which included the ferroelectric-to-paraelectric transition for LiNbO3. As we recently reported, at temperatures below almost-equal-to 1100 K, the probe substitutes primarily into the Li sites in both compounds. At higher temperatures, we find that the Hf-181 --> Ta-181 probe partitions between the Li and group-V sites. At these higher temperatures, the Li-site electric-field gradients (efg's) are characterized by high frequencies that are similar to those observed at lower temperatures. The group-V site efg's are characterized by frequencies that are approximately one tenth the magnitude of those corresponding to the Li sites. The occupancy of the Li site decreases and that of the group-V sites increases monotonically with increasing temperature. This partitioning is an equilibrium, reversible process that can be described by a simple thermodynamic model of second-order displacement. The temperature dependence of the associated equilibrium constant indicates enthalpies of several eV for the process.