Abstract
The lattice instability related to ferroelectric phase transitions of Bi3TiNbO9 and Bi3TiTaO9 has been studied by Raman scattering in the temperature range between 100 K and 1250 K. The lowest optical mode of Bi3TiNbO9 at 31 cm−1, softens remarkably upon heating towards the Curie temperature T c = 1223 K. Similarly, the lowest optical mode of Bi3TiTaO9 at 29 cm−1, also shows marked softening towards T c = 1143 K. The squared frequency changes linearly to temperature, and tends to zero at several hundred degrees above T c. The damping factor of these lowest modes increases markedly as their frequency decreases, and becomes overdamped at several hundred degrees below T c.
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