Structure and dynamics of the H0-tagged Li+ center in KCl as studied by polarized Raman scattering.

Abstract

Stabilization of a mobile interstitial hydrogen atom by a substitutional ${\mathrm{Li}}^{+}$ impurity in KCl leads to a ${\mathrm{H}}^{0}$${\mathrm{Li}}^{+}$ center possessing ${C}_{3v}$〈111〉 symmetry. A Raman line at 205 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is shown to be associated with this center. This line shows no isotope effect upon $^{6}\mathrm{\ensuremath{-}}^{7}$Li substitution. The behavior-type method developed recently by Zhou, Goovaerts, and Schoemaker was applied to the analysis of the polarized Raman spectra and it leads to the conclusion that the 205-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ line originates from an E mode. The inability to produce a non-random-orientational distribution at 4.2 K with 〈110〉-polarized light and the fact, deduced from electron-spin resonance, that the hydrogen maintains its free-atom character in the ${\mathrm{H}}^{0}$${\mathrm{Li}}^{+}$ center leads to the following picture. The small ${\mathrm{Li}}^{+}$ and the small ${\mathrm{H}}^{0}$ are both comfortably accommodated within the same cation site and each is displaced along 〈111〉 in opposite directions. There is no preferential molecular bond of any consequence between them. The well-established tunneling motion of the unperturbed 〈111〉-displaced ${\mathrm{Li}}^{+}$ center among its eight equivalent positions very likely persists in the ${\mathrm{H}}^{0}$${\mathrm{Li}}^{+}$ center and the ${\mathrm{H}}^{0}$ tags along while performing its own vibrational E mode.