Spectroscopic investigation of the ferroelectric phase transition in tris-sarcosine calcium chloride: an order-disorder system with displacive features. II. Soft modes in pseudo-spin-phonon coupled systems

Abstract

For pt.I see ibid., vol.17, p.3801 (1984). A model is developed for the softening of an optical phonon of frequency omega ₀ due to its interaction with a fast relaxator (a pseudo-spin of relaxation time tau ₀< omega ₀^-1). The short relaxation time of the pseudo-spin is achieved by thermally activated crossing of the barrier in a double-minimum potential starting from an excited vibrational level (vibration-enhanced barrier crossing). For comparison with Raman spectroscopic measurements of this soft mode the classical pseudo-spin and displacement correlation functions are calculated following the work of Yamada and co-workers (1974). The results are applied to the soft modes observed in tris-sarcosine calcium chloride (TSCC) and in various solid solutions of the brominated compound (TSCC_1-xB_x, O<or=x<or=0.7). The parameters obtained by fits of the correlation functions to the observed lineshapes are very reasonable. The relaxation time obtained compares favourably with the value determined from dielectric measurements.