Finite electric field effects on the incommensurate-commensurate transition in ferroelectrics

The influence of the external electric field on ferroelectrics exhibiting the incommensurate-commensurate phase transition is investigated within the Landau-type theory and the constant-amplitude approximation. The temperature variation of the modulation period, the polarisation and the dielectric susceptibility are calculated. It is shown that finite fields preserve the character of the transition, but lead to a shift of the transition temperature. A comparison with experiments on Rb2ZnCl is presented. The possibility of a field-induced Lifshitz point in incommensurate ferroelectrics is discussed.