Monoclinic Phase and Interphase Boundaries in Perovskite Ferroelectrics

The temperature intervals of phase coexistence, including the recent experimentally observed monoclinic phase, in ferroelectric BaTiO₃ have been investigated using the Landau-Ginzburg-Devonshire phenomenological approach. Comparison of the thermodynamic potential for all phases in BaTiO₃ showed that the monoclinic phase is metastable near the tetragonal-orthorhombic phase transition, in the absence of an electric field. The structure and width of the interphase boundaries separating domains of coexisting phases near the phase transitions have been analyzed. The temperature dependence of polarization distributions, width, and energy density of the interphase boundary separating the tetragonal and monoclinic phases as well as the orthorhombic and monoclinic phases were found. The obtained results for interphase boundaries were compared to previous experimental and theoretical results for the domain walls existing within the above-mentioned phases.