No abstract

A large ^-direction dependence of the broad band close to the TO peak is observed and explained qualitatively and quantitatively. The TO peak is found to be repelled out of a two-phonon band by a strong third-order anharmonic interaction.In a crystal having the zinc-blende structure, the first-order Raman scattering allows detection of either the TO(g~0) phonon or the LO(#~0) phonon, according to the selected direction q of the transferred momentum. We have measured at 40 K the scattered intensities of a CuCl single crystal for various q directions (Fig. 1). Because all the spectra were recorded with the same spectral slit width (2 cm" 1 ), the heights (eventually above the continuum) of the sharp lines at 171 and 208 cm" 1 could be taken to fix a common intensity scale for the drawings of Fig. 1 by making use of the fact that the chosen geometries impose I B = 0.5(JA+IC)-The unusual features in CuCl are the broad and strong band of the TO spectrum 1 " 3 (curve A) in the 140-170 cm" 1 range, reaching a very sharp line at 171 cm" 1 with a maximum at 151 cm" 1 , and its replacement by a weak and featureless continuum in the LO spectrum (curve C). This observation of ^-dependent, first-orderlike spectra obviously differs from the recent results of Shand et al. 3 and excludes the assignment of the broad band to a pure second-order Raman scattering.We propose to explain these features by the existence of strong anharmonic interactions between one of the <7~0 optical phonons and a twophonon continuum limited on the high-frequency side by a P 3 type of singularity 4 due to a Bril-where i labels the TO or LO phonon with frequency w,, R* y is the usual first-order Raman tensor element, the anharmonic third-order interaction being given by the usual formula 5 :(F 3 0 2 [A(a>) + iT(a>)]^ ^ TJ vfiirqJu -qiJ 2 ) G (u,qih, -Qi^ QsJJs* -(fa^M^', 4^3> -Qs^) • ( 2 ) q 3 ,J 3f i 4

Polarized Raman spectra of oriented single crystals of KCIO, performed over a large temperature range (300-800 K) have been used to investigate the mechanism of the 583 K phase transition. It is shown that the transition can be described using an order parameter and a mechanism based on the reorientation of the Clodions is proposed. A Raman study of the plastic phase (above 583 K), using the shape of the bands associated with the internal modes, leads to a measure of the orientational density probability and the orientational self-correlation functions of the perchlorate ions.