A Phenomenological Theory for Dielectric Anomaly near the Normal-Incommensurate-Commensurate Phase Transitions of Type-II

A phenomenological theory for dielectric anomaly near a lock-in transition of type-II has been developed. Temperature dependence of the dielectric constant has been calculated analytically. The effect of the gap between the coefficients of the different types of fourth-order term due to the long-range elastic force is considered. Due to the presence of this gap, the dielectric constant of a system described by a thermodynamic potential density where the coefficient of the fourth-order term is taken negative can exhibit a peak at the normal-incommensurate transition point. The temperature dependences of the anomalous heat capacity, the order parameter and the misfit parameter have been also calculated. The theory reproduces the features of the experimental data on NaNO 2 fairly well.