Lattice Dynamics of NaF

The results of an extended calculation of the normal modes of the NaF lattice are presented. Three models have been considered: a "rigid-ion" (RI) model, a "deformation-dipole" model with central ${\mathrm{F}}^{-}$-${\mathrm{F}}^{-}$ short-range interactions (DDNNN), and a "deformation-dipole" model including both central ${\mathrm{F}}^{-}$-${\mathrm{F}}^{-}$ interactions and a noncentral component in the first-neighbor, short-range interactions [DDNNN(NC)]. For each of these models we have derived frequency spectra for both 300 and 0°K input parameters, the associated dispersion curves for symmetry directions, Debye characteristic temperatures ${\Theta }_D$, moment functions ${\omega }_D\mathrm{}\left(n\right)\mathrm{}$, and the ${\mathrm{Na}}^{+}$ and ${\mathrm{F}}^{-}$ Debye-Waller parameters $B_1$ and $B_2$ and their ratio $\frac{B_1}{B_2}$. Of the three models, the two DD variations give good agreement with the observed dispersion curves, the DDNNN models being slightly better and giving somewhat closer agreement with the observed $B$ values.