Ab initio model potential embedded-cluster calculation of the geometric structure of T1(+) monomer and dimer centers in KCl

When alkali halide crystals are doped with ns? impurities such as Tl+, dime r centers as well as monomer centers are formed. In this paper, we present the results of ab initio model potential (AIMP) embedded cluster calculatio ns 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 calculat ion all the main interactions with a frozen lattice (classical Madelung and short-range Coulomb, quantum mechanical exchange and orthogonality) as wel l as the effects of polarization and relaxation of the lattice ions externa l to the cluster, we have optimized the geometry of clusters representing t he monomer and the D-2h and D-4h dimer defects, in their electronic ground state. We have found that the main result is an expansion of the clusters w ith 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, th e lattice relaxation effects result in an enhancement of the distortions ca lculated with a frozen lattice. Even though the structures of the present d efects are important ingredients for the derailed understanding of ns(2) de fects in halides, and in particular of their spectroscopy, no direct measur ements of them exist. Furthermore, the measurements are expected to be very difficult to perform, which leads to the relevance of the present calculat ions, The reliability of the results lies in the systematically good perfor mance that the present method observes in transition metal doped ionic crys tals. (C) 2001 Elsevier Science B.V. All rights reserved.