Glass Network Evolution with Bi3+/Ti4+ Substitution in Phosphate Glasses Formulated with a Constant Oxygen/Phosphorus Ratio. EXAFS, XANES, and 31P Double Quantum MAS NMR

Abstract

This paper reports a structural study of the effect of TiO2 substitution for Bi2O3 in xNa2O−xP2O5−yBi2O3−zTiO2 glasses, which were formulated in order to keep a constant average polymerization of the phosphate network. XANES and EXAFS analyses have indicated that the bismuth coordination sphere is similar to that described in other phosphate glasses [Daviero S.; Montagne L.; Palavit G.; Mairesse G.; Belin S.; Briois V. J. Phys. Chem. Solids 2003, 64 (2), 253], and that the titanium coordination sphere is asymmetric, but remains octahedral whatever the substitution degree. 31P double-quantum filtered MAS NMR shows that the Q2 fraction increases with the titanium content in the glasses, thus revealing a repolymerization of the phosphate network. The consideration of charges on oxygens (in terms of valence units) leads to the conclusion that titanium octahedra are coordinated to both Q1 and Q2 sites. The excess charge on these oxygens is compensated by Na+ removed from Q1 sites, which explains the observed repolymerization of the phosphate network. The evolution of glass properties is discussed within the context of this structural model. The increase of glass-transition temperature with titanium substitution for bismuth is due to the repolymerization of the phosphate network. The increase of electrical conductivity with titanium content is attributed to an increase of sodium mobility when it is involved in charge compensation of Ti4+ octahedra.