Electronic structure of fluoroborates activated by rare-earth elements

By means of a spin-polarized discrete variation method, electronic structure calculations have been performed for the optically active centers of rare-earth elements in the fluoroborates Mg{sub 3}BO{sub 3}F{sub 3}, Mg{sub 2}BO{sub 3}F, and Mg{sub 5}(BO{sub 3}){sub 3}F, which differ in fluorine content and crystal structure. Large fragments of the crystals, including a lanthanide atom, one or several fluorine centers, and three to five complete borate ions, have been examined. The transformation of AO populations and effective charges on the atoms in various systems have been investigated. Chemical bonding in the fluoroborates has been analyzed on the basis of overlap populations of orbitals of neighboring atoms. Excitation energies have been estimated for the matrices and impurity centers, and the influence of crystal structure on the spectral properties of these systems has been investigated.