Reorientation Motions of theHA(Li+)Center in KCl:Li+

Abstract

Through a combined electron paramagnetic resonance (EPR) and optical-absorption investigation, it is shown that the ${H}_{A}({\mathrm{Li}}^{+})$ interstitial center in KCl: ${\mathrm{Li}}^{+}$ exhibits two well-defined motions, the pyramidal motion (P. M.) and the restricted interstitial motion (R. I. M.). The P. M. possesses ${C}_{4v}$ symmetry around 100>: The ${\mathrm{Cl}}_{2}^{\ensuremath{-}}$ jumps around a given 100> direction among the four {110} half-planes. This thermally activated motion is responsible for the disorientation temperature ${T}_{D}(\mathrm{P}.\mathrm{M}.)=23.5$ K, around which all optical anisotropy produced at 4.2 K with 110> polarized light disappears, and for the lifetime broadening of the EPR lines above ${T}_{\mathrm{LB}}(\mathrm{P}.\mathrm{M}.)=75$ K. The P. M. is basically the same as the reorientation motions of the $H$ and ${H}_{{A}^{\ensuremath{'}}A}$ centers. The R. I. M. possesses ${C}_{3v}$ symmetry around 111>: The interstitial Cl exchanges molecular bonds with the three substitutional ${\mathrm{Cl}}^{\ensuremath{-}}$ ions that surround it. This motion is very similar to the ${V}_{K}$-center reorientation process. The thermally activated R. I. M. is responsible for the broadening of the EPR lines above ${T}_{\mathrm{LB}}(\mathrm{R}.\mathrm{I}.\mathrm{M}.)=29$ K, and at 75 K a motionally averaged ${\mathrm{Cl}}_{4}^{3\ensuremath{-}}$ EPR pattern of ${C}_{3v}$ symmetry is indeed observed. However, in the optical-anisotropy measurements no ${T}_{D}(\mathrm{R}.\mathrm{I}.\mathrm{M}.)$ is observed. It is concluded that the R. I. M. does not freeze in at low temperatures but goes over into a tunneling motion. As a result no optical anisotropy can be produced at 4.2 K with 100> polarized light. No differences have been observed so far for the R. I. M. and the P. M. between ${H}_{A}(^{6}\mathrm{Li}^{+})$ and ${H}_{A}(^{7}\mathrm{Li}^{+})$. A few comments are also made on the ${H}_{A}({\mathrm{Na}}^{+})$ center in KCl: ${\mathrm{Na}}^{+}$.