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Results of investigations of changes in the state of a ferroelectric crystal, through which an intense electromagnetic-energy flux is transmitted, are analyzed. It is found that the main parameters determining the phase transition change significantly in this case. In addition, some phenomena develop which are characteristic only of open thermodynamic systems (self-assembling dissipative structures arise). These phenomena manifest themselves upon generation of the second optical harmonic in crystals where natural matching is absent.

Although there is no natural phase matching or any phase matching structures, an optical second harmonic generation is observed in ferroelectric crystals such as BaTiO 3 and PbTiO 3 . It is shown, that the self-phase matching process is based on the second-order photorefraction, i.e. on the change of non-linear susceptibility X (2) versus the field intensity of the second harmonic electromagnetic wave. Thus, this change provides the quasi-phase matching.

Proceeding from the research of the second harmonics angular distribution, it is supposed that distortional deformations of a unit cell during spontaneous polarization plays an essential role in the formation of a microdomain structure in BaTiO 3 and PbTiO 3 crystals.

A new development in ferroelectricity is related to phenomena taking place in ferroelectrics and pyroelectrics with high carrier concentration (n > 1020 cm-3). The review is devoted to the thermal phenomena aspects, i.e. the heat capacity peculiarities, the thermoconductivity, the thermal expansion and thermo-e.m. power. The polar phenomena, related to charge and heat fluxes in the presence of spontaneous polarization, are especially predominant.

The new mechanism of charge and heat transfer in ferroelectric is theoretically and experimentally investigated. It is shown that below the phase transition the presence of abnormal electron and phonon mean values leads to the appearance of polar components of the thermocurrent and heat flow. These components are proportional to the spontaneous polarization vector and are independent of the temperature gradient direction.

It is shown that in the frame of the ferroelectric state model with electron-phonon coupling the temperature dependence of critical vibration and of order parameter can be explained by an electron-hole pair scattering on phonons. Such consideration does not permit phonon unharmonicity (independently from energy gap value) and considering the system instability and the system stabilization as a result of the same interaction. The connection of conduction-band and valence-band extreme mutual disposition with a phase transition type (ferroelectric, antiferroelectric, superstructural) is discussed.

A relation between absorption band frequencies connected with free carrier absorption the IR-region, mobility values, and phase transition temperatures in titanates, tantalates, and niobates is discovered. The existence of a single mechanism determining both phase transition and carrier kinetics in the conduction band is supposed. Es wird eine Beziehung zwischen den Frequenzen der Absorptionsbanden der freien Trager im infraroten Gebiet, der Grose der Tragerbeweglichkeiten und den Temperaturen der Phasenubergange in Titanaten, Niobaten und Tantalaten festgestellt. Das erlaubt zu vermuten. das ein einhcitlicher Mechanismus den Phasenubergang und die Kinetik der Trager im Leitungsband bestimmt.

A theory is developed of the bulk photovoltaic effect in ferroelectrics with strong interband interaction and of the coherent current state, connected with the interband absorption of electromagnetic field. The systematisation of possible nonlinear responses called by exciting of polar systems is suggested.

The behaviour of a ferroelectric in an intensive external electro-magnetic field is discussed. It is shown that, in fields with the amplitude E 0 > 105 V/cm, the phase transition temperature becomes higher, the static permittivity and Curie-Weiss constant increase. The optical absorption coefficient decrease changes to its increase. There is a threshold field, passing by which the system divides into layers of periodical structure of para-and ferroelectric phases. A difference for damping and refraction index was studied, while the field propagation was directed along and against the spontaneous polarization vector.

A number of phenomena are discussed which appear in the polar phase of nonequilibrium ferroelectric (systems without an inversion center): bulk photogalvanic effect, polar thermo-EMP and thermoconductivity and some others, in particular, these phenomena have anomalous and different values along and against to the spontaneous polarization velocity of distribution and damping of the electromagnetic and sound waves. All of them are connected with the specific character of the electronic quantum transitions in the system with off-diagonal long range order (ODLRO) and they can be united under one term “coherent effects”.

On an example of a two-band system with the electron-phonon interaction we show that a homogeneous stationary spontaneous current may arise when a phase transition in the presence of an external electromagnetic field occurs. The current is proportional to the imaginary part of the vector order parameter. In particular, in an illuminated homogeneous perfect ferroelectric this current appears simultaneously with the spontaneous polarization. The contribution of the spontaneous current into the photogalvanic (photovoltaic) effect is traced.

The light absorption and concentration dependence of the thermopower in BaTiO3 conducting single crystals were investigated. The absorption peak at Λ = 2 μm is interpreted as due to small polaron absorption. A quantitative comparison is made between theory and experiment. [Russian Text Ignored].

In a strong light fields the absorption and velocity of light are different when light propagating along and against the spontaneous polarization in ferroelectric. The first experimental results obtained by special polar photometers and interferometers are given.

Splitting of the mobility tensor component at the transition of BaTiO3 crystal from the cubic state to the tetragonal state is explained within the limits of the small-radius polaron model by the change of the overlapping integral magnitude. A quantitative comparison is made between theory and experiment and for this purpose the data obtained by Berglund and Baer, and Murakami were reproduced. From the same point of view Wemple's data about the dependence of mobility on hydrostatic pressure and a great difference of mobilities in SrTiO3 are considered.

Spontaneous increase in current in strong fields is found to be related to motion of color centers. Electron injection increases the number off, centers, which reduces the electrical strength.

The certain connection between Curie temperature and parameters of electronic spectrum (such as the energy gap, drift mobility, the position of the maximum of the intraband absorption of light) is discovered. The results are being discussed within the limits of the “pure interband” theory of ferroelectricity.

The anisotropy and the temperature dependence of the frequency and absorption value were measured in conducting BaTiO3 for the line connected with the polaron absorption (δ ∽ 2 μm). It is shown that the results can be explained within the limits of the small-radius polaron model.

Previous papers reported that the optical properties and conductivity of BaTi03 corresponds to the small polaron (SP) model. This report summarizes the literary and new experimental data about mobility μ and optical absorption spectra in the near IR-region for the crystals-analogs with different transition temperatures T tr (SrTi03, etc.) where the high conductivity was obtained. The absorption bands characteristic for SP were found not only in BaTi03 and SrTi03 but in PbTi03 and KNb03 as well. The table below shows that there is a correlation between μ, T tr and the energy corresponding to the absorption maximum .