Structural change of NaO1/2–WO3–NbO5/2–LaO3/2–PO5/2 glass induced by electrochemical substitution of sodium ions with protons

Structural changes of 35NaO1/2–1WO3–8NbO5/2–5LaO3/2–51PO5/2 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(P2O74−) chains on average and the terminal Q1 units (–O–PO33−) 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(P2O74−) 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.