Local lattice distortions around a Tl+-dimer substitutional impurity were investigated by using a mixed, first-principles and parametrized, methodology. An important conclusion was that explicit consideration of these distortions, for up to at least 4 coordination shells of ions around the impurity, was required in order to achieve convergent results for the first- and second-shell distortions. The lattice distortions introduced by D4h and D2h configurations were described, and their relative stabilities were analyzed by calculating the defect formation energies. It was found that the D4h configuration was lower in energy for both crystals. However, the energy difference was so small in the case of NaI that the 2 configurations could be considered essentially degenerate. It was also found that the inclusion of polarization interactions was essential for stabilization of the defect in NaI.

Local Structures and Relative Stabilities of Tl+-Dimer Substitutional Impurity Centers in NaI and KI. A.Aguado: Journal of Physics - Condensed Matter, 2001, 13[35], 8015-29