Ab initio model potential embedded-cluster calculation of the geometric structure of Tl + monomer and dimer centers in KCl

Abstract

When alkali halide crystals are doped with ns 2 impurities such as Tl +, dimer centers as well as monomer centers are formed. In this paper, we present the results of ab initio model potential (AIMP) embedded cluster calculations of the geometric structure of the centers formed in Tl +-doped KCl. Using the AIMP method, which is able to bring into an ab initio cluster calculation all the main interactions with a frozen lattice (classical Madelung and short-range Coulomb, quantum mechanical exchange and orthogonality) as well as the effects of polarization and relaxation of the lattice ions external to the cluster, we have optimized the geometry of clusters representing the monomer and the D 2 h and D 4 h dimer defects, in their electronic ground state. We have found that the main result is an expansion of the clusters with respect to the perfect lattice geometry, in line with the larger ionic radius of Tl + compared to K +. As it was observed in other point defects, the lattice relaxation effects result in an enhancement of the distortions calculated with a frozen lattice. Even though the structures of the present defects are important ingredients for the detailed understanding of ns 2 defects in halides, and in particular of their spectroscopy, no direct measurements of them exist. Furthermore, the measurements are expected to be very difficult to perform, which leads to the relevance of the present calculations. The reliability of the results lies in the systematically good performance that the present method observes in transition metal doped ionic crystals.