PARAMAGNETIC DEFECTS AS PROBES FOR THE STUDY OF FERROELASTIC PHASE TRANSITION IN LITHIUM NIOBATE AND LITHIUM TANTALATE UNDER HIGH PRESSURE.

It was found by optical polarization microscopy and the EPR study that lithium niobate and tantalate crystals undergo irreversible lattice changes under anisotropic hydrostatic compression. Regions having different cell orientations were registered. The observed changes were explained in terms of "strain switching" of ferroelastic domains. Possible sequence of phase transitions in these crystals ( Pm3m\leftrightarrow R\ \bar{3}\ c\leftrightarrow R\ 3\ c ) and the symmetry of the condensed soft modes ( R 25 and Γ 15 , correspondingly) were obtained by the analysis of the Gibbs free energy under external pressure. [ABSTRACT FROM AUTHOR]