Formation and Stability of Ferroelectric BaTi_2O_5

摘要

BaTi_2O_5 (BT_2) is thermodynamically stable over a very narrow temperature range between 1220° and 1230℃: a modification to the BaO-TiO_2 phase diagram is proposed. This thermodynamic stability was shown by constructing a time-temperature transformation diagram for the decomposition of BT_2. Once formed, BT_2 appears to be stable indefinitely at 1220°-1230℃; at higher temperatures, the decomposition rate increases with temperature; at lower temperatures, the decomposition rate increases with decreasing temperature and passes through a maximum at ～ 1200℃; below ～ 1150℃, BT_2 has long-lived kinetic stability. Kinetic considerations show a nucleation and growth mechanism for decomposition, with a nucleation induction period that is very temperature dependent. BT_2 can be prepared by various routes, including solid-state reactioin of oxides below ～ 1100℃; because it is metastable at all temperatures other than 1220°-1230℃, its formation is an example of Ostwald's rule of successive reactions. Discrepancies in the literature concerning the reported stability range of BT_2 can be explained by the complex dependence on temperature and time of both its formation and decomposition, for both of which, the nucleation stage is rate limiting.