It was noted that magnetic resonance data, on Cu2+-doped lattices with a fluorite structure, revealed that the equilibrium geometry for SrCl2:Cu2+ involved a large off-center displacement of Cu2+ along <001> type directions (Z0 ≈ 1.3Å). However, in CaF2:Cu2+ the impurity remained on-center; but with an orthorhombic distortion of the ligand shell. In the case of SrF2:Cu2+, an off-center motion was also observed but the estimated displacement (Z0 ≈ 0.8Å) was appreciably smaller than that for SrCl2:Cu2+. A microscopic explanation of these observations was attempted by performing calculations, within the framework of density functional theory, on clusters of up to 51 atoms. Total energy calculations, involving the application of the hybrid non-local B3LYP functional to clusters of only 21 atoms, were shown to reproduce reasonably well the main experimental features. These predicted Z0 = 0, 0.3 and 1.1Å for Cu2+-doped CaF2, SrF2 and SrCl2, respectively. A well depth of δ = 0.12eV, which was found for SrF2:Cu2+, was much smaller than that for SrCl2:Cu2+ (δ = 0.29eV) and was consistent with an incoherent hopping that was observed in the former case with increasing temperature and that led to an isotropic electron paramagnetic resonance spectrum. The main source of these surprising features was shown to be a competition between the electrostatic barrier, due to the rest of lattice ions acting upon the impurity ion, and the electronic energy gain that arose from bonding when the coordination number of the d9 impurity changed from 8 to 4. This showed that, for a given d9 ion, an increase in the host lattice parameter and a decrease in ligand electronegativity favored the occurrence of off-center motion. Consequent calculations, on SrX2:Mn2+ (X = F, Cl), gave Z0 = 0; in agreement with available electron paramagnetic resonance data. Contrary to the situation for Jahn–Teller distortions, off-center displacements were shown to be related to modifications of the related wave-functions, as described by the pseudo Jahn–Teller theory.

Cu2+ Impurities in Fluorite-Type Crystals - Mechanisms Favoring an Off-Center Motion. P.G.Fernández, J.A.Aramburu, M.T.Barriuso, M.Moreno: Physical Review B, 2004, 69[17], 174110 (8pp)