Quartic anisotropy of the recoilless fractions in NaCl

Previously obtained data on the retoilless fractions in single crystals of sodium chloride have been rsanalyzed in terms of the general formalism conf orming to the crystal symmetry. The original data were obtained by using Ra yleigh scattering of the Mossbauer radiation. It was found that above the q uantum region and at a temperature high enough to neglect anisotropy of the ionic form factors there is some anisotropy of the thermal motion of the e ntirely quartic character. On the other hand, quadratic terms in the wave-v ector transfer are completely quasiharmonic. A departure from the Gaussian thermal distribution is quite significant for both cations and anions. The anisotropy tends to diminish at high temperatures.