Solitons in Nonlinear Diatomic Lattices

We have obtained soliton solutions in the optical and acoustic branches by considering the long wavelength or continuum limit of diatomic lattices in three nonlinear potential regimes: cubic, both cubic and quartic, as well as fourth and sixth order nonlinearity in the nearest neighbour interactions. In the first two cases we get in the acoustic region KdV-type solitons and in the optical region nonlinear Schrödinger type amplitude dulated envelope solitons, where as in the later case we get MKdV-type acoustic solitons and tanh-kink-antikink optical solitons. These tanh-kink solitary waves can be used to account for the structural phase transition in perovskite ferroelectrics.