Lattice Dynamics of Sodium Fluoride

Abstract

Phonons in sodium fluoride have been studied at room temperature using inelastic neutron scattering techniques. Consistent results were obtained using a cold neutron time-of-flight apparatus and a triple-axis spectrometer. The time-of-flight results were interpolated on to symmetry directions from the observed scattering surfaces. The frequencies in units of ${10}^{12}$ ${\sec }^{-1\mathrm{}}$ of some typical phonons are: TA(0,0,1), $\nu =4.39\mathrm{}\pm 0.04$; LO(0,0,0.984), $\nu =8.52\mathrm{}\pm 0.15$; LA(0,0,0.972), $\nu =7.94\mathrm{}\pm 0.18$; TO(0.488, 0.488, 0.488), $\nu =6.19\mathrm{}\pm 0.07$. The optical branches extrapolated to small wave vector are in agreement with the infrared absorption frequency and the Lyddane-Sachs-Teller relation. Hardy and Karo's deformation-dipole model is in agreement with the results to within 6%, but the rigid-ion model differs by as much as 19%. The results are well fitted by a shell model containing nine parameters in which the ionic charge is 0.91.