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, T P P B = 43 C ∘ , for the BZT-40BCT system, and provides a maximum temperature of coexistence, T C , 0 = 49.9 C ∘ , in agreement with experiments. For T > T P P B , 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 T P P B , 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.