Electron paramagnetic resonance (EPR) spectra of Nb4+ ions in Li2O-Nb2O5-P2O5 glasses with different composition of oxide components have been investigated. The EPR spectrum shape analysis of Nb4+ (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 Nb4+ ions in glasses reversibly changes with temperature and is explained via the mechanism of electron hopping between niobium ions in clusters. The dependence of the amount of Nb4+ ions upon Li2O content has a maximal character, which implies that small amounts of Li+ ions stabilize the Nb4+ 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 on model binuclear complexes, (HO)(3)Nb-O-Nb(OH)(3), [(HO)(3)Nb-O-Nb(OH)(3)]Li+, and [(HO)(3)Nb-O-Nb(OH)(3)](Li+)(2) that exhibit the reversible disproportionation reaction Nb4+-O-Nb(4+)double left right arrowNb(3+)-O-Nb5+. Triplet states of these complexes (total electron spin S = 1) have lower energies than singlet 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 theoretical modeling to the analysis of the experimental EPR spectrum in Li2O-Nb2O5-P2O5 glass concludes noncentrosymmetric structure of binuclear niobium complex with similar to0.1 Angstrom offset of the bridging oxygen atom towards one Nb atom. (C) 2003 American Institute of Physics.