RANDOM-FIELDS AND THEIR INFLUENCE ON THE PHASE-TRANSITIONS IN DISORDERED FERROELECTRICS

Calculations of the phase transition temperature and the critical conc entration of electric dipoles which induce in the host crystal lattice the phase transition with the appearance of long-range order are carr ied out. These calculations are made by considering the additional sta tic random electric fields owing to defects of other types: elastic di poles and point electric charges. The distribution function of the joi nt action of the aforementioned random fields is also calculated. It i s shown that the presence of the defects of other types inhibits phase transition so that, for the occurrence of long-range order, in this c ase larger concentrations of electric dipoles am necessary. The theore tical results obtained are found to be important for a description of the experimental situation in K1-xLixTaO3, PbMg1/3Nb2/3O3 and other di sordered ferroelectrics.