NONSOLITON MODEL FOR DIELECTRIC ANOMALIES AT THE INCOMMENSURATE-FERROELECTRIC PHASE-TRANSITIONS IN [N(CH3)4]2XCL4 (X=CO,ZN)

The dielectric peaks observed at the phase transition between an incommensurate- and a commensurate-ferroelectric phase have usually been explained in terms of a soliton-regime model of the incommensurate phase. However, in the case of [N(CH3)4]2XCl4 (X=Co, Zn), recent observations provide evidence for a strong coexistence of both the incommensurate and ferroelectric phases. This behavior seems to discard a proper soliton regime as a model for the phase transition. We show here that in fact the dielectric behavior of these compounds can be explained assuming that the incommensurate modulation remains approximately in a sinusoidal configuration in the whole temperature range of interest. In accordance with the experimental results, the model considers that the transition between the incommensurate phase (in a sinusoidal regime) and the ferroelectric phase has a first-order character. A divergency of the dielectric susceptibility of [N(CH3)4]2CoCl4 and [N(CH3)4]2ZnCl4 is predicted as the incommensurate modulation (always in a sinusoidal regime) passes through the commensurate value q=2/5c*. The obtained divergency law differs from the usual Curie-Weiss law and is enough to reproduce in general terms the dielectric anomaly observed in [N(CH3)4]2CoCl4. In this comparison we use as input data the temperature dependence of the wave vector and the coexistence ratio of incommensurate and ferroelectric phases, which were determined in some recent x-ray measurements. The present model considers a modulated order parameter in the frame of a generalized Landau analysis, but it is the low-temperature threefold-ferroelastic commensurate phase that is taken as the reference for the order parameter. Previous incommensurate and commensurate phases are described by an adequate modulation of the order parameter. A generalization of this approach to any incommensurate system exhibiting successive into different commensurate phases is also discussed. © 1990 The American Physical Society.