Acoustic absorption by the soft modes of defects in NH4Cl

2014 Intrinsic defects tend to oscillate relative to the domains in the two-phase region of the orientational transformation. When the solid is treated as a continuous elastic medium, the vibrational frequencies of vacancies and mobile dislocations are found to decrease as (To T)1/3 and | T0 T |1/2, respectively, near the transition point To. From the ultrasonic excitation of the soft mode of dislocations at angular frequency 03C9 the internal friction acquires displaced maxima at To ± k03C92 and extended tails of the form 03C9 | T0 T|-1. A spectroscopic study of the transitional opalescence is proposed to check the estimate of 50 MHz as a lower bound on the frequency of the soft mode of dislocations in a good crystal. J. Physique LETTRES 41 (1980) L-161-L-164 leI’ AVRIL 1980,