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

Electron paramagnetic resonance (EPR) spectra of ${\mathrm{Nb}}^{\text{4+}}$ ions in ${\mathrm{Li}}_2\mathrm{O}–{\mathrm{Nb}}_2{\mathrm{O}}_5–{\mathrm{P}}_2{\mathrm{O}}_5$ glasses with different composition of oxide components have been investigated. The EPR spectrum shape analysis of ${\mathrm{Nb}}^{\text{4+}}$ (electron configuration ${\text{4d}}^1,$ electron spin $\text{S=1/2)}$ reveals the formation of triplet niobium binuclear complex (total electron spin $\text{S=1)}$ in glasses. The amount of ${\mathrm{Nb}}^{\text{4+}}$ 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 ${\mathrm{Nb}}^{\text{4+}}$ ions upon ${\mathrm{Li}}_2\mathrm{O}$ content has a maximal character, which implies that small amounts of ${\mathrm{Li}}^{+}$ ions stabilize the ${\mathrm{Nb}}^{\text{4+}}$ pairs, but cause their disproportionation at higher concentrations of ${\mathrm{Li}}^{+}$ ions in the glass. Quantum mechanical analysis of electronic and spin states of binuclear niobium clusters has been performed on model binuclear complexes, $(\mathrm{HO}{)}_3\mathrm{Nb}–\mathrm{O}–\mathrm{Nb}(\mathrm{OH}{)}_3,$ $\mathrm{[(}\mathrm{HO}{)}_3\mathrm{Nb}–\mathrm{O}–\mathrm{Nb}(\mathrm{OH}{)}_3]{\mathrm{Li}}^{+},$ and $\mathrm{[(}\mathrm{HO}{)}_3\mathrm{Nb}–\mathrm{O}–\mathrm{Nb}(\mathrm{OH}{)}_3\mathrm{](}{\mathrm{Li}}^{+}{)}_2$ that exhibit the reversible disproportionation reaction ${\mathrm{Nb}}^{\text{4+}}–\mathrm{O}–{\mathrm{Nb}}^{\text{4+}}\iff {\mathrm{Nb}}^{\text{3+}}–\mathrm{O}–{\mathrm{Nb}}^{\text{5+}}.$ Triplet states of these complexes (total electron spin $\text{S=1)}$ have lower energies than singlet states $\text{(S=0),}$ and ${\mathrm{Li}}^{+}$ 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 ${\mathrm{Li}}_2\mathrm{O}–{\mathrm{Nb}}_2{\mathrm{O}}_5–{\mathrm{P}}_2{\mathrm{O}}_5$ glass concludes noncentrosymmetric structure of binuclear niobium complex with ∼0.1 Å offset of the bridging oxygen atom towards one Nb atom.