Title: Several possible phase states in oxide ferroelectrics

Abstract

The analysis of phase diagrams, structure and crystal lattices dynamics of oxide ferroelectrics of lithium niobate and tantalate at various concentrations of topological defects was carried out. The structure and dynamics of the lattice in the region optical vibrations were studied by the methods of X-ray structural analysis, quasi-elastic and Raman light scattering spectroscopy. Sound absorption of longitudinal and shear waves and Q-factor of samples were obtained by the radio engineering method and by the Bragg light scattering on sound method. Three types of lithium niobate samples were prepared for the studies - those grown from a stoichiometric melt, grown from a congruent melt and grown from a congruent melt with different rates of pulling out the seed from the melt in order to create different concentrations of dislocations. We calculated the intensity of Raman light scattering in the frequency range 5 - 70 cm-1 at the model interaction of 2A1 (TO) symmetry optical vibration with acoustic vibrations relaxing on dislocations. The self-energy part of acoustic phonons was calculated from the Langevin equation. Good agreement of the calculated spectrum with the experimental one was obtained. The high degree of correlation between the parameters makes it possible to dispense with only one parameter in the used model - the concentration of topological defects in the samples. It is shown that there is a critical value of the concentration of topological macrodefects, at which a correlation occurs between dislocations.