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

We have studied the interstitialcy mechanism for interstitial cation d iffusion in Li2O and LiCl. First, we have checked and validated the ap plicability of the ionic approximation and the reliability of the pair -potential technique for the investigation of interstitial cation diff usion in ionic crystals. For these purposes we calculated the electron ic structure and optical absorption energies for these ions, and compa red the atomic structure calculated quantum-chemically with that calcu lated employing the pair-potential technique. Secondly we have shown t hat the interstitialcy mechanism is very effective because of the rela tively small additional displacements of ions on moving from the initi al configuration to the barrier point in compact sublattices, such as the cation sublattice in Li2O or in LiCl. This results from the favour able lattice distortion around the interstitial ion. In addition we re port a comparison of results obtained using the MOLSTAT Mott-Littleton code on CaF2 with earlier calculations that employed the HADES progra m, and obtain a satisfactory measure of agreement between the two sets of calculations.