Relationship between structure and mobility of proton carriers injected by electrochemical substitution of sodium ions with protons in 35NaO_1/2-1 WO₃-8NbO_5/2-5LaO_3/2-51PO_5/2-based glasses

Using Raman and ³¹P magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopies, we studied the structural changes in 35NaO_1/2-1WO₃-8NbO_5/2-5LaO_3/2-51PO_5/2 glass upon introducing Al₂O₃ and/or Y₂O₃ in order to understand the reduced mobility of proton carriers in glasses with Al₂O₃ and/or Y₂O₃ in which proton carriers were injected by alkali-proton substitution (APS). The Raman and ³¹P MAS-NMR spectra showed that phosphate chains were shortened by the Al₂O₃ and/or Y₂O₃ introduced into 1W glass. This structural change increased the fraction of protons bound to oxygen atoms in the terminal PO₄ (i.e., the Q¹ unit) of the phosphate chains. Because the protons bound to terminal Q¹ units in phosphate chains tightly bind to oxygen atoms, as opposed to the protons bound to inner Q² units in phosphate chains, we attributed the reduced mobility of proton carriers upon introducing AlO_3/2 and/or YO_3/2 into 1W glass to the shortening of phosphate chains. From these results we propose that to obtain a highly proton-conducting glass after APS, the glass must have a composition of O/P < 3.5 and thus a sufficiently high fraction of Q² units.