Theory and mechanism for martensitic transformations in pervoskite ferroelectrics

Abstract

A theory of martensitic transformations is applied to structural changes in pervoskite ferroelectrics whereby the transformation is accomplished by two lattice-variant shears; (i) a primary shear on a low index system which is relatively soft, has degenerate stress-sense, and transforms all vectors in a certain plane (the habit plane) to their final values, and (ii) a habit plane shear which completes the structural change and accomodates the volume change by a relaxation normal to the habit plane. The primary shear, with its degenerate stress-sense, periodically alternates its direction to satisfy the “minimum long range stress field criterion” and “minimum depolarization field criterion”, with the result that the product, for the cubic→tetragonal transformation, consists of twinned 90° domains of alternating polarization. The theory, applied to various perovskite ferroelectrics and antiferroelectrics, gives insight into dynamical and critical behavior; these are discussed.