Temperature Dependence of Infrared Dispersion in Ionic Crystals LiF and MgO

Infrared-reflectivity measurements were made from 200 ${\mathrm{cm}}^{-1\mathrm{}}$ to 800 ${\mathrm{cm}}^{-1\mathrm{}}$ at temperatures ranging from 7.5 to 1060°K for LiF and from 8 to 1950°K for MgO. The reflection spectra were analyzed by means of a two-resonance damped-oscillator model, and the calculated optical properties are presented. Dielectric dispersion theory is reviewed and it is shown that all the major theories discussed give identical results for the susceptibility when evaluated at the reststrahlen frequency but differ from one another at other frequencies. The damping constant $\gamma$ for LiF and MgO in the high-temperature limit ($h{\nu }_1<\mathrm{kT}$) agrees reasonably well with the formula suggested by Maradudin and Wallis, but discrepancies seem to appear in the low-temperature limit ($h{\nu }_1>\mathrm{kT}$). The general behavior of the extinction coefficient in the wings of the absorption region is consistent with the notion of continuous absorption produced by multiphonon processes. The shift of the long-wavelength optical-mode frequencies of LiF, MgO, and RbI with temperature is discussed in terms of the volume and other anharmonic effects. The anharmonic part of the frequency shift is found to agree qualitatively with the theory of Maradudin and Fein.