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

Through a combined electron paramagnetic resonance (EPR) and optical-absorption investigation, it is shown that the $H_A\left({\mathrm{Li}}^{+}\right)$ 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 : The ${\mathrm{Cl}}_2^{-}$ jumps around a given direction among the four {110} half-planes. This thermally activated motion is responsible for the disorientation temperature $T_D(\mathrm{P}.\mathrm{M}.\mathrm{})=23.5\mathrm{}$ K, around which all optical anisotropy produced at 4.2 K with polarized light disappears, and for the lifetime broadening of the EPR lines above $T_{\mathrm{LB}}(\mathrm{P}.\mathrm{M}.\mathrm{})=75\mathrm{}$ K. The P. M. is basically the same as the reorientation motions of the $H$ and $H_{A^{\prime }A}$ centers. The R. I. M. possesses $C_{3v}$ symmetry around : The interstitial Cl exchanges molecular bonds with the three substitutional ${\mathrm{Cl}}^{-}$ 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 $T_{\mathrm{LB}}(\mathrm{R}.\mathrm{I}.\mathrm{M}.\mathrm{})=29\mathrm{}$ K, and at 75 K a motionally averaged ${\mathrm{Cl}}_4^{3-}$ EPR pattern of $C_{3v}$ symmetry is indeed observed. However, in the optical-anisotropy measurements no $T_D(\mathrm{R}.\mathrm{I}.\mathrm{M}.\mathrm{})$ 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 polarized light. No differences have been observed so far for the R. I. M. and the P. M. between $H_A\left({}_{}{}^6{}_{}{}^{}\mathrm{Li}_{}^{+}{}_{}{}^{}\right)$ and $H_A\left({}_{}{}^7{}_{}{}^{}\mathrm{Li}_{}^{+}{}_{}{}^{}\right)$. A few comments are also made on the $H_A\left({\mathrm{Na}}^{+}\right)$ center in KCl: ${\mathrm{Na}}^{+}$.