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Relaxor behavior and morphotropic phase boundary in a simple model

A simple model to reproduce strong enhancement of dielectric response near the morphotropic phase boundary (MPB) is proposed. This model consists of long-range dipole-dipole interaction and compositional chemical disorder incorporated by the variation in lengths of dipole moments. By applying Monte Carlo simulation, we show that there appears a ferroelectric boundary phase between two types of antiferroelectric phases at an optimal strength of randomness. In the boundary phase, ferroelectric domain becomes remarkably large and flexible to external electric fields, leading to huge dielectric response. This observation indicates that huge dielectric response near the MPB originates from {\it local} polarization rotation under suppressed anisotropy by phase competition.