Impurity-Induced Raman Scattering in SrF2and BaF2

Abstract

Measurements have been made of the impurity-induced Raman scattering spectra due to ${\mathrm{Eu}}^{2+}$ impurities substituting for the cations in Sr${\mathrm{F}}_{2}$ and Ba${\mathrm{F}}_{2}$ single crystals. It is found that the data cannot be fitted to a nearest-neighbor coupling model as has been used previously to fit similar data in some alkali-halide hosts. An electron-phonon coupling model is presented in which purely Coulombic coupling is assumed so that the range of the coupling may be extended beyond the nearest neighbors without introducing new adjustable parameters. As a point-charge model is found insufficient to account for the relative intensities in the regions of acoustic- and optic-phonon scattering, extended charge distributions are assumed for the ionic shells. This treatment gives a qualitative fit to the gross intensity distribution of the observed spectra. However, the approximations used in the model result in some discrepancies between the calculated and observed spectra. It is concluded that the nearest-neighbor coupling model will give an adequate fit to the impurity-induced spectra only in hosts in which all ions in the unit cell vibrate with approximately equal amplitudes in the optic-phonon region.