Phase coexistence near the polymorphic phase boundary

Abstract

A novel multiphase field theory for ferroelectric systems in the vicinity of a polymorphic phase boundary (PPB) is developed by coupling the Landau-Devonshire thermodynamic potentials of the individual phases. The model naturally predicts metastable coexistence of the rhombohedral (R) and tetragonal (T) phases near the PPB temperature, TPPB=43C∘, for the BZT-40BCT system, and provides a maximum temperature of coexistence, TC,0=49.9C∘, in agreement with experiments. For T>TPPB, results show that metastable coexistence of two ferroelectric phases is a result of a phase transformation-induced polarization rotation plus switching mechanism. Metastable domains of the low-temperature R phase coexist with the high-temperature, thermodynamically stable T phase for long periods of time, from minutes to hours. For T<TPPB, the coexistence time is on the order of tens of seconds due to a decreased thermal energy that suppresses the polarization rotation plus switching mechanism. Further, the kinetics of macroscopic T→R phase transformation is accelerated by a large thermodynamic driving force and high mobility.