Electron paramagnetic resonance study of W5+ pairs in lithium-tungsten phosphate glasses

The electron paramagnetic resonance (EPR) spectra of W5+ ions in Li2O-WO3-P 2O5 glasses with different composition of oxide components have been invest igated. The anisotropy of the axially symmetric g factor decreases with an increase of Li content in the glass and with an increase in temperature. Ev idence for the formation of W5+ ion pairs has been obtained by using a dual -mode EPR cavity that allows measuring EPR spectra with polarization of the magnetic vector B-1 of the microwave perpendicular or parallel to the stat ic magnetic field B-0. Enhancement of the relative intensity of the half-fi eld DeltaM(S) = 2 spin transition was observed at B-1\\B-0. The ratio ct of the intensity of the DeltaM(S) = 2 transition to the intensity of the Delt aM(S) = 1 transition was used to describe the effectiveness of pair formati on between W5+ ions in the glass and to determine the mean distances r betw een W5+ ions. The parameter alpha increases with an increase of Li2O conten t in the glass. This shows that the inclusion of Li+ ions favors the format ion of tungsten pairs. The mean distance between W5+ ions decreases from r = 4.53 Angstrom for the glass without Li+ ions to r = 3.70 Angstrom for the glass containing 50 mol % Li2O. Disproportionation of W5+ ions in the pair is thermally reversible in the presence of Li+ ions such that temperature decrease shifts the equilibrium toward the formation of W4+ and W6+ ions.