X-ray scattering by phonons in sodium fluoride

Abstract

Measurements of the absolute intensity of the x-ray thermal scattering from sodium fluoride crystals have been made at a representative set of positions in reciprocal space. The distribution of scattered intensity can be interpreted to give the ionic form factor dependence on neighbouring ionic positions during thermal motion. The simplest interpretation is a fractional change of the form factor of a negative ion with relative displacement of a nearest-neighbour positive ion, increasing with the magnitude of the scattering vector K. The measurements also give transverse and longitudinal phonon dispersion curves for the [1,0,0], [1,1,0] and [1,1,1] directions. The errors in the frequencies are small except for a few higher-frequency phonons. The slopes of the dispersion curves at low wave vectors suggest that the differences between the first- and zero-sound velocities are less than 3%.