Relaxation variation of the nature of the phase transition in a ferroelectric semiconductor

Abstract

The second-order ferroelectric phase transition, which is close to the Lifshitz point and the tricritical point on the phase diagram, is shown to evolve in time into a sequence of second- and first-order transitions. As the temporal variations of the temperature dependence of the dielectric constant of the ferroelectric semiconductor Sn2P2S6 suggest, the exposure of the crystal at a fixed temperature in the para phase near the second-order phase transition point gives rise to an intermediate state (presumably incommensurate). Illuminating the sample promotes the formation of such a state, while a constant electric field directed parallel to the spontaneous polarization axis narrows the temperature range within which such a state exists. Qualitatively the experimental data are described by the mean-field approximation within a model that assumes the coefficients of the thermodynamic potential of a uniaxial intrinsic ferroelectric with a single direction of modulation in the incommensurate phase to be linear functions of the charge carrier concentration on the trapping level, which varies with time exponentially as the system relaxes to the equilibrium state at a fixed temperature.