Relaxational change of the phase transition character in ferroelectric-semiconductor

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Abstract
The effect of time evolution of second-order ferroelectric phase transition (PT), which is close to the Lifshitz point and the tricritical point in the phase diagram, to the sequential second-order and firstorder transitions has been found. As the data on time variations of the temperature dependence of dielectric constant for Sn2P2S6 ferroelectric-semiconductor show, the crystal exposure at fixed temperature in the paraphase in the vicinity of the second-order PT results in the appearance of an intermediate (incommensurate, hypothetically) state. Sample lighting benefits the formation of such state, whereas a constant electric field directed along the spontaneous polarization axis reduces its temperature range. Experimental data are described qualitatively by mean-field approximation within a framework of a model, that suggests the linear dependence of thermodynamic potential coefficients of the uniaxial proper ferroelectric with a single direction of modulation in the incommensurate phase vs the charge carriers concentiation on the trapping level which is varied exponentially with time in the process of system relaxation to the equilibrium state at fixed temperature.

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