Elastic Properties and the Phase Transitions in Hexagonal Barium Titanate

Elastic properties of haxagonal BaTiO 3 have been investigated through measurements of elastic compliances by the composite-bar method. At the structural-phase-transition temperature T a ∼215 K, the effective elastic compliance is found to exhibit a stepwise anomaly. Phenomenological theory of this phase transition is given by using a model that a silent E 2u mode in the high-temperature hexagonal phase might be the soft-phonon mode of which frequency vanishes at T a . At another transition temperature T c ∼54 K, measured elastic compliances are found to exhibit a divergent increase. Together with other experimental evidences, the phase transition at T c is interpreted to be of a ferroelastic nature.