The Thermodynamics of Photoferroelectrics

Abstract

Phase transitions in solids are now believed to be associated with instability of one of the vibration modes. In particular, ferroelectric phase transitions are associated with the unstable (‘soft’) optical mode at the center of the Brillouin zone. While the behavior of the electron spectrum and electron-phonon interactions have practically been ignored until recently, a large body of experimental data has been accumulated on the effect of electrons, and of nonequilibrium electrons in particular, on ferroelectric phase transitions and the properties of ferroelectrics well below the Curie point. The effect of nonequilibrium electrons on phase transitions and properties of ferroelectrics is of great interest from the experimental standpoint. Phase transitions due to such electrons are described as photostimulated in the literature, and the phenomena associated with the effect of nonequilibrium electrons (and of electron excitations generally) on the ferroelectric properties, as photoferroelectric [2.1–4]. Thus, we shall use the term photoferroelectrics to refer to crystals which change their ferroelectric properties (or properties connected with the spontaneous polarization in one way or another) when optically excitated in the intrinsic or extrinsic region of the spectrum.