On some problems of the phenomenological theory of ferroelectric-semiconductors

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Abstract
Thermodynamic potentials of a ferroelectric-semiconductor (F-S) are represented in the form of the known Landau-Ginsburg-Devonshire expansion, taking into account the concentrations of free or excited electrons and the mechanical and electromechanical invariants of the series. The form of the thermodynamic function has been defined for the case of an intrinsic and extrinsic F-S having a many-valleyed conduction band with electrons in equilibrium and for a photoelectric with nonequilibrium electrons in the bands and on local centres.The necessity of taking into account linear mechanical invariants in the above-mentioned cases is emphasized. The thermodynamic description of the phase transition in F-S is compared with theories in which the ferroelectric transition is the result of the interband electron-phonon interaction connected with the changes in the number of filling of bands with carriers. The effect of the electrons is considered on the character of the ferroelectric transition (Curie point shifts, photohysteresis, photostrain etc.), as well as the change of the energy gap width as function of temperature, field and mechanical stresses in the case of photoferroelectric when illuminated. Some peculiarities of the domain structure of F-S are also considered.

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