First-principles super-cell investigation of the rattling effect in Li-doped KCl

We have studied by the first-principles total energy method the off-center instability of a substitutional Li impurity in KCl. We report here the results of super-cell calculations of the energy associated with displacing the Li along [Formula: see text], [Formula: see text] and [Formula: see text] directions relative to the K vacancy. To understand the influence of relaxations, we performed three levels of relaxations-only first nearest neighbors of the vacancy, first and second nearest neighbors, and full relaxation. Our calculations show that the relaxation significantly affects the energy surface and that the relaxation of the nearest neighbors to the vacancy accounts for most of the trend. The position of minimum energy for the Li was found to be along the [Formula: see text] direction. The calculated well along [Formula: see text] is 102 meV deep relative to the on-center energy and lies about 0.86 Å off-center. Minima along adjacent [Formula: see text] directions are separated by a saddle-point barrier of 44 meV along the [Formula: see text] direction. Our calculated results are in agreement with prior model calculations and we believe our calculated potential surfaces are the best theoretical result so far.