Fluctuation effects in a model of displacive ferroelectrics

The self-consistent phonon field method is used to investigate the phase transition in a model of a displacive ferroelectric. It is shown that the zeroth approximation of the method takes into account the fluctuations of the order parameter and is equivalent to the renormalized Landau expansion of the free energy. An iteration procedure is proposed for calculating the order parameter, this taking into account systematically the principal terms of the second order of the theory.