PHASE-TRANSITIONS IN DISORDERED FERROELECTRICS WITH 2 TYPES OF RANDOMSITE ELECTRIC-DIPOLE

Phase transitions in a highly polarizable crystal of cubic symmetry in duced by two types of random site electric dipole orientated correspon dingly along the [100]- and [111]-type directions are considered. The distribution function of random electric fields produced by these dipo les was calculated. This function made it possible to obtain the order parameters, the critical concentrations of dipoles and the transition temperatures for ferroelectric phase transitions with tetragonal and rhombohedral symmetries. All these quantities were calculated as funct ions of the ratio of dipole moments and the concentrations of the two types of dipole considered. It was shown that, at certain concentratio ns and dipole moment ratio, the order parameters of both aforementione d ferroelectric phases had to coexist. The mean values of random elast ic fields produced by the random electric fields considered through th e electrostriction effect are calculated. It was found that in crystal s such as KTaO3 the elastic fields of E(g) symmetry may be larger than those of T-2g symmetry. The eight-well adiabatic potential of the clu ster, which includes the impurity dipole and its nearest neighbours, w as shown to be transformed into a six-well potential under the action of E(g) symmetry deformations. This transformation was supposed to be the main reason for the tetragonal symmetry phase transitions observed earlier in K1-etaLietaTa1-xiNbxiO3 (eta less than or equal to 0.06; x i less than or equal to 0.028) even at very low lithium ion concentrat ions. The calculation of the transition temperature confirmed this sup position and made it possible to explain qualitatively the transition temperature concentrational dependence observed in the aforementioned mixed disordered system.