Since Valasek's discovery of the ferroelectric properties of Rochelle salt nearly 60 years ago, ferroelectricity has been regarded as one of the tradi­ tional branches of dielectric physics. It has had important applications in lattice dynamics, quantum electronics, and nonlinear optics. The study of electron processes in ferroelectrics was begun with VUL's investigations of the ferroelectric properties of barium titanate [1.1]. In­ trinsic and extrinsic optical absorption, band structure, conductivity and photoconductivity, carrier mobility. and transport mechanisms were examined in this compound, and in other perovskite ferroelectric semiconductors. An important discovery was that of the highly photosensitive photoconducting ferroelectrics of type AVBVICVIII (e.g. SbSI) by MERZ et al. in 1962 [1.2,3]. A large number of ferroelectric semiconductors (some photosensitive, some not) are now known, including broad-band materials (e.g. lithium niobate, lithium tantalate, barium and strontium niobate, and type-A~B~I compounds), BI and narrow-band semiconductors (e.g. type_AIVB compounds). A series of improper ferroelectric semiconductors and photosensitive ferroelastics have been discovered, of which Sb 0 I is an example. s 7 Owing to the uncertainty of their band structure, the difficulty in deter­ mining the nature of the levels, the complexity of alloying, and their gen­ erally low mobility values, ferroelectrics are rarely of interest regarded as nonlinear semiconductors. The most fruitful approach has been the study of the influence of electrons (especially nonequilibrium electrons) and electron excitations on phase transitions and ferroelectric properties. A large group of phenomena have recently been discovered and investigated.

1. Introduction.- 2. The Thermodynamics of Photoferroelectrics.- 2.1 The Free Energy of a Ferroelectric Semiconductor.- 2.2 First- and Second-Order Phase Transitions.- 2.3 Effect of Electrons on Phase Transitions: Photoferroelectric Phenomena.- 2.4 The Energy-Gap Anomaly.- 2.5 Electrical Conductivity of Ferroelectric Semiconductors Near the Curie Point.- 3. The Microscopic Theory of Photoferroelectric Phenomena.- 3.1 Ferroelectric Phase Transitions and the Soft Vibration Mode..- 3.2 Effect of Screening on the Soft Vibration Mode.- 3.3 Ferroelectric Phase Transitions and Interband Electron-Phonon Interactions.- 3.4 Change in Dipole Moment of Centres on Optical Recharging.- 3.5 Phasons and Fluctuons.- 4. Screening of Spontaneous Polarization.- 4.1 Debye Length as Screening Length Parameter in Ferroelectrics.- 4.2 Screening and Periodic Structure of Interphase Boundaries in Ferroelectrics.- 5. Photoferroelectric Phenomena and Photostimulated Phase Transitions.- 5.1 Thermodynamics of Photoferroelectric Phenomena.- 5.2 Photostimulated Shift of Phase Transition Temperature and Photohysteresis Effect.- 5.3 Influence of Electron Excitations on Spontaneous Polarization.- 5.4 Photodeformation Effect.- 5.5 Photoinduced Rayleigh Scattering in BaTiO3.- 6. The Anomalous Photovoltaic Effect in Ferroelectrics.- 6.1 The APV Effect in Ferroelectrics.- 6.2 Photovoltaic Current with Short-Circuited Electrodes.- 6.3 The Nature of the AP Effect in Ferroelectrics.- 7. The Photorefractive Effect in Ferroelectrics.- 7.1. Mechanisms of the PR Effect in Ferroelectrics.- 7.2. Photorefractive Holographic Recording.- 7.3 Photorefractive Sensitivity.- 7.4 Effect of Photorefraction on Multiphoton Excitation.- 8. Screening Phenomena.- 8.1 Influence of Nonequilibrium Carriers on Screening of Interfaces.- 8.2 Influence of Screening on Switching Processes.- 8.4 Screening and Short-Circuit Photocurrents.- References.