Volnianskii D. Dielectric properties of crystals in Li2Ge4O9-LiNaGe4O9 system.

Object are the ferroelectric single crystals of Li2-хNaхGe4O9 (0,2 < х < 0,3). The aim is to study of the main peculiarities of the polarization effects in Li2-хNaхGe4O9 crystals under the action of electric fields and temperature. Metods - growth of Li2-хNaхGe4O9 crystals by Czochralsky technics, measurements of dielectric characteristics by the bridge method, tecnics to study switching processes in ferroelectrics in alternative and constant electric fields. It was no possibility to obtain crystals with 0 < х < 0,2 by Czochralski technique. Single crystals of Li2-хNaхGe4O9 (0,2 < х < 0,3), with the phase transition temperature from 270 up to 335 K were grown at the first time. It is shown, that nonlinear dielectric properties of crystals with the Tc < 300 K may be well described within the framework of Landau thermodynamic theory for the second order phase transitions, but the property of crystals with the Tc > 300 K are not described by this theory. In samples with the Tc < 300 K usual P-E hysteresis loops are observed. Double Р-Е hysteresis loops are observed in limited interval of temperatures in the ferroelectric phase in samples with Tc > 300 K. The model that explains the appearance of double P-E loops by the reorganization of the domain structure in these crystals is suggested. This model is proved to be true by the measurements of pyroelectric current dependencies on crystals with Tc < 300 K and Tc > 300 K. Sphere of use are ferroelectric single crystals, training courses.