X-Ray Study of the Lattice Dynamics of Sodium Chloride

Abstract

Measurements have been made of the thermal diffuse x-ray scattering from sodium chloride. We describe our analysis of these results to give phonon frequencies in the [111] and [100] directions, and discuss the factors limiting the accuracy of the x-ray method. The frequencies are in good agreement with the theory for alkali halides in the dipole approximation, but not with the theory for rigid ions. At long wavelengths they are consistent to 2% with ultrasonic measurements, and also with the longitudinal optic frequency predicted from the Lyddane-Sachs-Teller relation. A new feature in the x-ray diffuse scattering has also been observed which, it is suggested, results from the dependence of the scattering factors of the ions on the nuclear coordinates.