Acoustic-phonon dispersion at incommensurate phase transitions

Abstract

This paper comprises three lectures given at the School on Acoustic Phonons in Varna, Bulgaria, from September 23 to October 2, 1982. Anomalous dispersion of sound velocities, both transverse (TA) and longitudinal (LA), is discussed for ferroelectric insulators near their incommensurate/commensurate phase-transition temperatures. In BaMnF4 the LA dispersion has characteristic time 10-10s and is attributed to amplitudons; the LA frequency behavior satisfies the defect theory of Hochli and Bruce. The TA dispersion has characteristic time 10-9s and is attributed to phasons. The anomalous temperature dependence of the pyroelectric coefficient near T1 in BaMnF4 reported by Negran is also discussed and related to specific-heat and sound-velocity behavior. Results are summarized for BaMnF4, Ba2NaNb5O15, NaNO2, SC(NH2)2, RbH3(SeO3)2, K2SeO4 and Rb2ZnCl4.