INFLUENCE OF THE RANDOM ELASTIC FIELDS ON THE PHASE-TRANSITIONS IN DISORDERED FERROELECTRICS

The calculation of the electric dipoles critical concentration in the disordered ferroelectics with the random site elastic dipoles is carried out. The crystals with electrostriction and piezoelectricity in paraelectric phase containing the elastic dipoles with an arbitrary ratio of their dilatation and quadrupole parts are considered. Two cases of the quadrupole axis orientation, namely parallel and transverse to spontaneous polarization direction have been taken into account. It was found that the critical concentration dependence on elastic dipoles concentration is the nonlinear one in the crystals with electrostriction while it is linear in the crystals with piezoelectricity in paraelectric phase. The numerical estimations for KTaO3 have shown that the several percents of the elastic dipoles can destroy the ferroelectric phase transition induced by electric dipoles. The distribution function of the random electric fields induced both by the electric and elastic dipoles, which are independent defects in lattice, is calculated for all the aforementioned cases.