Off-CenterLi+in KCl

The minimum-energy configuration of the ions surrounding a substitutional ${\mathrm{Li}}^{+}$ impurity in KCl is determined using a point-ion model. The position of minimum energy for the ${\mathrm{Li}}^{+}$ itself was found to be off the lattice site in a $〈111〉$ direction. The potential which a ${\mathrm{Li}}^{+}$ ion sees in its translational tunneling between neighboring minima is given as a symmetrical sixth-power polynomial. The tunneling splitting and dipole moment are found to be in reasonable agreement with experiment. The Born-Mayer-Verwey potential is tried and rejected because of its unreasonable hardness and a potential appropriate to the experimental data is obtained. Uniform-hydrostatic-pressure effects are estimated, and the possibility of other small impurity ions being off-center is discussed. A substitutional ${\mathrm{Li}}^{+}$ in KBr is also considered.