Dispersion of elastic moduli and order parameter fluctuations in ferroelectric Rochelle salt

Rochelle salt is piezoelectric both in the polar and nonpolar phases. We have investigated the quasilongitudinal phonon propagating along 1√2(0,1,1) which is coupled linearly to the order parameter, the polarization component P1. The temperature dependence of phonon frequency, phonon linewidth and central component can be explained quantitatively by means of the piezoelectric coupling to the order parameter which is known to undergo a Debye relaxation. In the ferroelectric phase (monoclinic) additional elastic moduli arise. By measuring their dispersion these morphic moduli are shown to be due only to electrostriction.