Incommensurate-Commensurate Transitions in Solid and Liquid Crystalline Ferroelectrics in External Fields

The phase diagrams of proper and improper incommensurate solid ferroelectrics in external electric fields and of ferroelectric liquid crystals in external magnetic fields are reviewed. It is shown that the Landau model for solid improper ferroelectrics with n=4 is isomorphous to the model for ferroelectric liquid crystals in an external magnetic field allowing for the existence of a Lifshitz point. The experimental results are reviewed and a new model for ferroelectric liquid crystals is proposed.