Polarized Raman study of phonon modes perturbed by the off-center Li+ impurity in KCl.

Abstract

In KCl:${\mathrm{Li}}^{+}$ crystals a Raman-active resonance associated with the ${\mathrm{Li}}^{+}$ off-center impurity was observed. The Raman signal is a broad line with a full width at half maximum of 15 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and is centered at 43\ifmmode\pm\else\textpm\fi{}2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which is almost coincident with the far-infrared transitions of this defect. The behavior-type method was applied to determine the symmetry and the irreducible representation of this Raman mode. It is found to belong to the ${A}_{1g}$ representation of the cubic group ${O}_{h}$. This strongly indicates that a continuum of perturbed lattice modes is observed, and not the vibration in an off-center potential well of the ${\mathrm{Li}}^{+}$ ion itself. The small and negative isotope shift upon $^{7}\mathrm{Li}^{+}$ to $^{6}\mathrm{Li}^{+}$ substitution also supports this idea. As such, the ${C}_{3v}$ symmetry of the off-center ${\mathrm{Li}}^{+}$ ion in KCl is not reflected in the polarized Raman intensities of the 43-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode. However, the displacement from the on-center site softens the selection rules of scattering and absorption and this explains the approximate degeneracy of the Raman and ir transitions.