EFFECT OF UNIAXIAL PRESSURE ON THE 222-K STRUCTURAL PHASE-TRANSITION OF HEXAGONAL BATIO3

The temperature dependence of the c-axis dielectric constant (epsilon-c) of hexagonal BaTiO3 under uniaxial pressure has been measured in the vicinity of the structural phase transition at T(a) = 222 K for pressure applied along several different crystal axes. Uniaxial compression in the c plane affects the transition temperature and also induces a dielectric anomaly around the shifted transition temperature, while stress along the c axis does not. The effect of uniaxial pressure is analyzed in terms of a phenomenological free-energy expression involving the E2u soft phonon mode. The increase of the dielectric anomaly produced by either [100] or [110] stress is interpreted as a pressure-induced improper ferroelectricity. From these experimental results, the coupling coefficients in the Gibbs free energy have been determined. From these experimental results, the coupling coefficients in the Gibbs free energy have been determined. The characteristics of the multicriticality and the nonclassical crossover behavior of the T(a) phase transition are discussed. Phase diagrams in the stress-temperature planes are presented. In addition, we predict the possibility of an additional pressure-induced phase transition from mm2 to 222 for pressure applied along the [100] axis.