Structural change of NaO_1/2-WO₃-NbO_5/2-LaO_3/2-PO_5/2 glass induced by electrochemical substitution of sodium ions with protons

Structural changes of 35NaO_1/2-1WO₃-8NbO_5/2-5LaO_3/2-51PO_5/2 glass (1W-glass) before and after the electrochemical substitution of sodium ions with protons by alkali-proton substitution (APS) are studied by Raman and ³¹P magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopies. The glass before APS consists of (PO₃^-)_8.6(P₂O₇^4-) chains on average and the terminal Q¹ units (-O-PO₃^3-) are bound to MO₆ octahedra (M denotes niobium or tungsten) through P-O-M bonds. Some non-bridging oxygens (NBOs) in the MO₆ octahedra are present in addition to the bridging oxygens (BOs) in P-O-M bonds. APS induces fragmentation of the phosphate chains because the average chain length decreases to (PO₃^-)_3.7(P₂O₇^4-) after APS, despite the total number of modifier cations of sodium and lanthanum ions and protons being unaffected by APS. This fragmentation is induced by some of the NBOs in the MO₆ octahedra before APS, changing to BOs of the newly formed M-O-P bonds after APS, because of the preferential formation of P-OH bonds over M-OH ones in the present glass. We show that APS under the conditions used here is not a simple substitution of sodium ions with protons, but it is accompanied by the structural relaxation of the glass to stabilize the injected protons.