Abstract
A review is given of the experimental and theoretical study of the effect of off-center Li and Nb ions in mixed ferroelectric crystals K1-xLixTaO3 and KTa1-yNbyO3. EPR line broadening is analyzed in detail for paramagnetic centers which show a linear or a quadratic Stark effect in an external electric field. It is shown that a body of experiments on axial and cubic centers Fe3+ are consistent with the model of ferroelectric phase transition driven by off-center ions induced polar nanoregions above Tc. The size of these nanoregions as well as their reorientational frequency and effective dipole moment of off-center ions are obtained from the EPR data.
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