Effect of uniaxial stress on the $H_A\left({\mathrm{Li}}^{+}\right)$ center in KCl:${\mathrm{Li}}^{+}$: Change of center geometry by stress

With no applied external uniaxial stress the ${\mathrm{Cl}}_2^{-}$ axis of the $H_A\left({\mathrm{Li}}^{+}\right)$ center in KCl:${\mathrm{Li}}^{+}$ makes a 26° angle with [100] in the (110) plane. It is shown through an analysis of the electron-paramagnetic-resonance spectra that [001] uniaxial stress changes the geometry of the $H_A\left({\mathrm{Li}}^{+}\right)$ center at low temperatures. The ${\mathrm{Cl}}_2^{-}$ axis moves out of the (110) plane and with increasing stress describes an octant of a cone around [100] having a 2×26° = 52° apex angle. At high uniaxial stresses (∼ 5×${10}^8$ dyn/${\mathrm{cm}}^2$) and at 4.2 K the ${\mathrm{Cl}}_2^{-}$ axis is within a few degrees of the (001) plane. Raising the temperature above 4.2 K counteracts the effect of the uniaxial stress while lowering the temperature aids it. Above ∼50 K the [100] uniaxial stress is no longer effective in changing the $H_A\left({\mathrm{Li}}^{+}\right)$ center geometry. The stressed $H_A\left({\mathrm{Li}}^{+}\right)$ center possesses a restricted interstitial motion (RIM) which, similar to the RIM of the unstressed $H_A\left({\mathrm{Li}}^{+}\right)$ center, may be tunneling a low temperatures.