Anharmonic Mode-Mixing in Ionic Ferroelectrics,

The effects of various anharmonic interactions between the soft mode and other modes in an ionic ferroelectric have been examined, with a view of predicting sudden temperature variations in the frequencies of the latter. The normal coordinates and frequencies of the deformation modes even modes connected with electrostriction change at the transition points in a way depending on whether the anharmonic coupling is of the third or of the fourth order. The longitudinal optical mode when coupled to the transverse mode changes discontinuously in the way observed by Perry and Hall, provided an isotropic coupling is assumed however, the continous variation of the longitudinal frequency with temperature is not given by the mechanism. Author