Phenomenological Theory of Antiferroelectric Transition. IV. Ferrielectric

Abstract

Kittel’s free energy expression with all the fourth order cross terms of two sublattice polarizations was dealt with. Phase diagrams of four phases, the nonpolar, polar, antipolar, and semipolar phase, were calculated for various sets of expansion coefficients. For a set of coefficients, the semipolar phase is absolutely stable at zero field, and it is switched to the polar phase by external field indicating ferrielectric behaviors. Rather complicated relations of polarization and external field were calculated systematically corresponding to phase transitions among the four phases.