Electron paramagnetic resonance and quantum-mechanical analysis of binuclear niobium clusters in lithium-niobium phosphate glaases

摘要

Electron paramagnetic resonance (EPR) spectra of Nb~(4+) ions in Li_2O - Nb_2O_5-P_2O_5 glasses with different composition of oxide components have been investigated. The EPR spectrum shape analysis of Nb~(4+) (electron configuration 4d~1, electron spin S = 1/2) reveals the formation of triplet niobium binuclear complex (total electron spin S = 1) in glasses. The amount of Nb~(4+) ions in glasses reversily changes with temperature and is explained via the mechanism of electron hopping between niobium ions in clusters. The dependence of the amount of Nb~(4+) ions upon Li_2O content has a maximal character, which implies that small amounts of Li~+ ions stabilize the Nb~(4+) pairs, but cause their disproportionation at higher concentrations of Li~+ ions in the glass. Quantum mechanical analysis of electronic and spin states of binuclear niobium clusters has been performed no model binuclear complexes, (HO)_3Nb-O-Nb(OH)_3, [(HO)_3Nb-O-Nb(OH)_3]Li~+, and [(HO)_3Nb-O-Nb(OH)_3](Li~+)_2, that exhibit the reversible disproportionation reaction Nb~(4+) -O-Nb~(4+) <=> Nb~(3+)-O-Nb~(5+). Triplet states of these complexes (total electron spin S = 1) have lower energies than single states (S = 0), and Li~+ ions stabilize the binuclear niobium complex. We have found that electron spin densities on niobium ions change depending upon the shift of the bridging oxygen atom. Application of this thoretical modelling to the analysis of the experimental EPR spectrum in Li_2O-Nb_2O_5-P_2O_5 glass concludes noncentrosymmetric structure of binuclear niobium complex with approx 0.1 A offset of the bridging oxygen atom towards one Nb atom.