Lattice model for elastic ferroelectric crystals: Microscopic approach

A lattice model is presented for ferroelectric crystals with molecular groups. The model is one dimensional, diatomic, and endowed with permanent microscopic dipoles which give rise to an additional rotational degree of freedom. The latter, in turn, yields the ferroelectric soft mode which couples with the acoustic modes. In the model the interactions between all the elements of the chain are taken into account. This leads to the study of the stability of the chain, which can be connected with the notion of phase transition. The analysis of linear wave motions exhibits the repulsion between the rotational and either transverse or longitudinal acoustic modes according to whether the permanent microscopic dipoles are initially aligned with the chain or perpendicular to it.