Lattice distortions around a Tl(+) impurity in NaI:Tl(+) and CsI:Tl(+) scintillators: An ab initio study involving large active clusters

Ab initio perturbed ion cluster-in-the-lattice calculations of the impurity centers NaI:Tl 1 and CsI:Tl 1 are presented. We study several active clusters of increasing complexity and show that the lattice relaxation around the Tl 1 impurity implies the concerted movement of several shells of neighbors. The results also reveal the importance of considering a set of ions that can respond to the geometrical displacements of the inner shells by adapting self-consistently their wave functions. Comparison with other calculations involving comparatively small active clusters serves to assert the significance of our conclusions. Contact with experiment is made by calculating absorption energies. These are in excellent agreement with the experimental data for the most realistic active clusters considered. @S0163-1829~98!09741-0#