Phonon Dispersion Relations in NaCl

Phonon dispersion relations in NaCl at 80^ have been determined in the crystallographic 100, 101, and 111 symmetry directions using inelastic neutron scattering. Some measurements have also been made at 300^, and at (1. 0, 0. 5, 0) in reciprocal space. The results are compared with theoretical calculations: those of Karo and Hardy for their "deformed-dipole" model, those of Schr\"oder and N\"usslein for a "breathing-shell" model, and those of Caldwell and Klein for a shell model. Satisfactory agreement is obtained in many places, but significant discrepancies still remain, particularly close to the zone boundaries. Comparison is also made with earlier x-ray work on NaCl. The work was performed on a new crystal spectrometer at the R2 reactor at Studsvik, which is equipped with a two-crystal monochromator.