Role of internal stress on room-temperature permittivity of BaTiO3 ceramics and thin films

Anomalies observed in the room-temperature permittivity of BaTiO3 (BTO) ceramics and thin films with second phases were investigated. In BTO ceramics with slight Ba-excess composition, Ba2TiO4 particles were found and the overall sample permittivity increased, compared to stoichiometric BTO. For BTO films with Ti-excess composition, Ti-excess intergranular phase was found and the permittivity was significantly lower than that of the stoichiometric film. Transmission electron microscopy revealed that the BTO ceramics/films were under high stress due to the thermal mismatch with these second phases. To explain the different behaviors of permittivity in two cases, Landau–Devonshire theory was applied, based on a geometric model depicting the different state of stress between BTO matrixes and second phases. The calculation showed that the permittivity change predicted by the theory was consistent with experimental results. This study illustrates the effect of the geometry of the second phase and thermal stress on ferroelectric properties.