Cu2+ impurities in fluorite-type crystals:: Mechanisms favoring an off-center motion -: art. no. 174110

Magnetic resonance data on Cu2+-doped lattices with fluorite structure reveal that the equilibrium geometry for SrCl2:Cu2+ involves an important off-center displacement of Cu2+ along <001> type directions (Z(0)approximate to1.3 Angstrom) while in CaF2:Cu2+ the impurity remains on-center though with a orthorhombic distortion of the ligand shell. In the case of SrF2:Cu2+ an off-center motion is also observed though the estimated displacement (Z(0)approximate to0.8 Angstrom) is substantially smaller than that for SrCl2:Cu2+. A microscopic explanation of these challenging facts has been attempted through calculations in the density functional theory framework on clusters involving up to 51 atoms. Total energy calculations using the hybrid nonlocal B3LYP functional on clusters of only 21 atoms are shown to reproduce reasonably main experimental features leading to Z(0)=0, 0.3, and 1.1 Angstrom for Cu2+-doped CaF2, SrF2, and SrCl2, respectively. The well depth delta=0.12 eV obtained for SrF2:Cu2+ is substantially smaller than that for SrCl2:Cu2+ (delta=0.29 eV) and is consistent with the incoherent hopping observed in the former case upon temperature raising leading to an isotropic EPR spectrum. The origin of these surprising facts is shown to come mainly from a competition between the electrostatic barrier due to the rest of lattice ions upon the impurity ion and the electronic energy gain from bonding when the coordination number of the d(9) impurity changes from 8 to 4. This simple argument explains that keeping the same d(9) ion, an increase of the host lattice parameter and a decrement of ligand electronegativity favor the occurrence of an off-center motion. Following these ideas calculations on SrL2:Mn2+ (L=F, Cl) give Z(0)=0 in agreement with available EPR data. At variance with what happens for Jahn-Teller distortions off-center displacements are shown to be related from the beginning to modifications of involved wavefunctions as described by the pseudo Jahn-Teller theory.