Local structures and relative stabilities of Tl+-dimer substitutional impurity centres in NaI and KI

The local lattice distortions around a Tl+-dimer substitutional impurity in NaI and KI have been investigated by using, a mixed ab initio/parametrized methodology. One important conclusion of the work is that an explicit consideration of these distortions up to at least the first four coordination shells of ions around the impurity is needed in order to achieve a converged result for the first- and second-shell distortions. After describing the lattice distortions induced by both D-4h and D-2h configurations, we analyse their relative stability by calculating the defect formation energies. It is found that the D4h configuration is lower in energy for both crystals, but the energy difference is so small in the case of the NaI host lattice that the two configurations can be considered degenerate for all practical purposes. It is also found that inclusion of polarization interactions is crucial in the stabilization of the defect in NaI.