Infrared absorption and Raman scattering ofOH−andOD−stretching-mode vibrations in alkali fluorides: Isolated defects

Abstract

The stretching-mode (SM) of ${\mathrm{OH}}^{\ensuremath{-}}$ and ${\mathrm{OD}}^{\ensuremath{-}}$ defects in LiF, NaF, and KF hosts has been studied with Raman scattering and Fourier transform infrared (FTIR) absorption spectroscopy, at temperatures from 4 K to 500 K. In all three hosts the $〈111〉$ orientation of the ${\mathrm{OH}}^{\ensuremath{-}}$ molecular axis is confirmed both by polarized Raman spectra and by stress-induced FTIR dichroism. The latter technique also yields the values of the disc-shaped elastic dipole tensor, aligning the ${\mathrm{OH}}^{\ensuremath{-}}$ parallel to an applied $〈111〉$ uniaxial stress with a Curie law. Accurate band-shape analysis of low-temperature FTIR absorption and ${T}_{2g}$ Raman spectra yields for low concentrations of ${\mathrm{OH}}^{\ensuremath{-}}$ in LiF and KF consistent values of their nearest-neighbor reorientational tunneling parameters $(\ensuremath{\Delta}=1.2$ and $0.6{\mathrm{cm}}^{\ensuremath{-}1},$ respectively). For ${\mathrm{OH}}^{\ensuremath{-}}$ in NaF and for ${\mathrm{OD}}^{\ensuremath{-}}$ in all three hosts spectral substructure from tunneling is undetectable in the narrow Lorentzian-shaped SM lines. The strong temperature broadening that occurs for the SM absorption and ${T}_{2g}$ Raman bands and the smaller broadening of the ${A}_{1g}$ line can be fitted with models for the dephasing of the transition by phonons, considering the presence of isotropic and anisotropic dephasing components. In spite of the very different amplitudes of the ${\mathrm{OH}}^{\ensuremath{-}}$ and ${\mathrm{OD}}^{\ensuremath{-}}$ SM vibrations, their relative infrared absorption and Raman intensities are found to be very similar; the only exception is an extremely weak oscillator strength of the ${\mathrm{OD}}^{\ensuremath{-}}$ absorption in KF.