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

Abstract

The electron paramagnetic resonance (EPR) spectra of W 5+ ions in Li 2 O−WO 3 −P 2 O 5 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 W 5+ 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 static magnetic field B 0 . Enhancement of the relative intensity of the half-field Δ M S = 2 spin transition was observed at B 1 || B 0 . The ratio α of the intensity of the Δ M S = 2 transition to the intensity of the Δ M S = 1 transition was used to describe the effectiveness of pair formation between W 5+ ions in the glass and to determine the mean distances r between W 5+ ions. The parameter α increases with an increase of Li 2 O content in the glass. This shows that the inclusion of Li + ions favors the formation of tungsten pairs. The mean distance between W 5+ ions decreases from r = 4.53 Å for the glass without Li + ions to r = 3.70 Å for the glass containing 50 mol % Li 2 O. Disproportionation of W 5+ ions in the pair is thermally reversible in the presence of Li + ions such that temperature decrease shifts the equilibrium toward the formation of W 4+ and W 6+ ions.