Microscopic theory of lattice instability in displacive type ferroelectrics

Abstract

A new first principle approach to the problem of ferroelectric instability is advanced. This model-free approach permits to investigate the role of dipolar forces in the origin of ferroelectric instability in crystals of arbitrarily low ionicity possessing finite but anyhow small dielectric gap width in the electronic spectrum. In this approach, a contribution of dipolar forces to the frequencies of longwave transversal polar optic modes is deduced from the macroscopic quantities accessible from the experimental data. Some applications to IV-VI compounds and ferroelectric perovskites are presented.