Polarization-flip phase transitions under an electric field in displacive systems with competing periodicities

Abstract

As an alternative to the usual Landau-Ginzburg-type continuous field approximation, a free energy featuring two anticrossed phonon branches and defined in a discrete lattice is proposed for dealing with modulated phases with inherent discrete lattice effects. Using this approach, it is shown that, at some threshold electric (or conjugate) field, phases may appear in the phase diagram of solids having several modulated phases of different periods. These phases are the result of the sign-reversal of a particular polar local mode in a spinlike modulation of the order parameter, and have indeed been recently observed in the modulated ferroelectric betaine calcium chloride dihydrate (BCCD). This type of field-driven polarization-flip transition had not been anticipated by previous theoretical approaches, including microscopic ones such as the ANNNI model. The model proposed also explains the quite peculiar topological features of the phase diagram under the electric field observed in BCCD.