THERMAL LIGHT SCATTERING BY IONIC CRYSTALS.

The temperature and crystal orientation dependence of the thermal scattering by ionic crystals is studied in the temperature range 78 to 300 degrees Kelvin, with the following results. The absolute magnitude of the intensity, the temperature dependence in first approximation, and the shift in wavelength from the incident wavelength of the components of the ideal crystal Brillouin scattering all agree with theory. The defect scattering increases linearly with temperature in first approximation. The depolarization ratios rho sub V are approximately zero. The defect scattering as a function of the crystal orientation angle theta has 2-fold symmetry. These defect scattering results are explained in terms of clouds of point-like scattering units vacancies around charged edge dislocations with the dislocations aligned approximately parallel to a single 100 direction in the crystal. The core of the dislocation, a region of great disorder, acts as a source and sink of vacancies and the change in the number density of the vacancies with temperature leads to the observed temperature dependence. The measurement of the scattered light as a function of the angle theta allows the intensity of the true, ideal crystal, Rayleigh scattering and the Landau-Placzek ratio for sodium chloride to be determined for the first time. The results are obtained with the essential help of a new detection technique which eliminates the stray background light without using the immersion technique or special crystal geometries. Author