Vibrational properties versus on- and off-axis behavior of the FA(Li+) center in alkali halides.

Abstract

The polarized resonant Raman spectra of the ${\mathit{F}}_{\mathit{A}}$(${\mathrm{Li}}^{+}$) center in several alkali halides are interpreted by means of a behavior-type analysis. This yields the defect symmetry and the representation of the observed vibrational modes. It is found that in a given host lattice the on- or off-center behavior of the isolated ${\mathrm{Li}}^{+}$ center is systematically preserved on perturbing it with a nearest-neighbor F center. The Raman spectra of the off-axis ${\mathit{F}}_{\mathit{A}}$(${\mathrm{Li}}^{+}$) centers exhibit both the ${\mathrm{Li}}^{+}$ vibrations perpendicular and parallel to the defect axis. For the on-axis centers, only the latter is observed. The occurrence of high-frequency ${\mathrm{Li}}^{+}$ modes for both on- and off-axis centers is related to a displacement of the ${\mathrm{Li}}^{+}$ equilibrium position parallel to the defect axis and, due to the small impurity size, away from the adjacent F center. The occurrence of high-frequency ${\mathrm{Li}}^{+}$ modes is attributed to ionic relaxation of the impurity and/or its nearest neighbors. This is in agreement with the theoretically calculated high-frequency ${\mathrm{Li}}^{+}$ modes for the off-center ${\mathrm{Li}}^{+}$ impurity in KCl and with the observation of low-frequency localized modes for the isolated ${\mathrm{Li}}^{+}$ impurity with on-center behavior. The frequencies of the lowest vibrational levels in the off-axis system RbCl:${\mathit{F}}_{\mathit{A}}$(${\mathrm{Li}}^{+}$) are very close to those of the harmonic approximation. This is in contrast to the strongly anharmonic ${\mathrm{Li}}^{+}$ potential, which was established for the isolated on-center ${\mathrm{Li}}^{+}$ in KBr.