CALCULATION OF ADIABATIC BARRIERS FOR CATION DIFFUSION IN LI2O AND LICL CRYSTALS

We have studied the interstitialcy mechanism for interstitial cation diffusion in Li2O and LiCl. First, we have checked and validated the applicability of the ionic approximation and the reliability of the pair-potential technique for the investigation of interstitial cation diffusion in ionic crystals. For these purposes we calculated the electronic structure and optical absorption energies for these ions, and compared the atomic structure calculated quantum-chemically with that calculated employing the pair-potential technique. Secondly we have shown that the interstitialcy mechanism is very effective because of the relatively small additional displacements of ions on moving from the initial configuration to the barrier point in compact sublattices, such as the cation sublattice in Li2O or in LiCl. This results from the favourable lattice distortion around the interstitial ion. In addition we report a comparison of results obtained using the MOLSTAT Mott-Littleton code on CaF2 with earlier calculations that employed the HADES program, and obtain a satisfactory measure of agreement between the two sets of calculations.