Ferroelectric phase transitions: A review of theory and experiment

Whereas in X-ray and neutron diffraction we examine the static ionic displacements which accompany the appearance of spontaneous polarization, together with the real parts of frequencies of excitations which propagate through the crystal, we are here concerned with the dynamic mechanisms which relax fluctuations from equilibrium and render the phonon frequencies complex. The effect of a transition on these relaxation processes is to increase greatly the relaxation time and attendant sound absorption and, in some crystals, to produce an anomalous decrease in the velocity of transverse waves. The absorption and velocity are normally measured by passing a pulse in the megaycle range through the sample and then observing the transit time and the decay of the amplitude for successive echos.