Temperature Dependence of Long-Wavelength Optic Phonons of NaF Single Crystals

Infrared reflection spectra of NaF single crystal from 160 to 660 cm^-1 as functions of temperature up to about 1000 ^∘{}K are presented. A Kramers-Kronig (KK) dispersion analysis is applied to these spectra. The optical constants, long-wavelength optic-phonon frequencies (both TO and LO), and their half-widths are obtained as functions of temperature. The room-temperature KK results are in aggreement with thin-film transmission, oblique incident reflection measurements, and earlier reported data. The general behavior of anharmonic effects in NaF is discussed and compared with data of other ionic crystals as well as some theoretical predictions.