Electron Paramagnetic Resonance Study of W⁵⁺ Pairs in Lithium−Tungsten Phosphate Glasses

Abstract

The electron paramagnetic resonance (EPR) spectra of W5+ ions in Li2O−WO3−P2O5 glasses with different composition of oxide components have been investigated. The anisotropy of the axially symmetric g factor decreases with an increase of Li content in the glass and with an increase in temperature. Evidence 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 B1 of the microwave perpendicular or parallel to the static magnetic field B0. Enhancement of the relative intensity of the half-field ΔMS = 2 spin transition was observed at B1||B0. The ratio α of the intensity of the ΔMS = 2 transition to the intensity of the ΔMS = 1 transition was used to describe the effectiveness of pair formation between W5+ ions in the glass and to determine the mean distances r between W5+ ions. The parameter α increases with an increase of Li2O content in the glass. This shows that the inclusion of Li+ ions favors the formation of tungsten pairs. The mean distance between W5+ ions decreases from r = 4.53 Å for the glass without Li+ ions to r = 3.70 Å 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.