Phase transitions in BaTiO₃ under uniaxial compressive stress: Experiments and phenomenological analysis

The relative permittivity of polycrystalline BaTiO₃ was measured from –150 °C to 250 °C at compressive bias stresses up to −500 MPa. Mechanical loading shifted the rhombohedral-orthorhombic, orthorhombic-tetragonal, and tetragonal-cubic phase transition temperatures and produced a pronounced broadening of the dielectric softening in the vicinity of all three transitions. The inter-ferroelectric rhombohedral-orthorhombic and orthorhombic-tetragonal phase transitions were found to be less stress sensitive than the ferroelectric-paraelectric transition occurring between tetragonal and cubic phases at the Curie point. The application of compressive stress resulted in a strong suppression of the relative permittivity, such that at the highest applied stress of −500 MPa, the permittivity in the single phase regions away from the phase transitions was found to display only a weak dependence on temperature between −100 °C and 125 °C. The experimental observations closely followed the predictions of a 2-4-6 Landau polynomial wherein the dielectric stiffness and higher-order dielectric stiffness coefficients are linear functions of uniaxial stress.