31P NMR in (100 - x)(NaPO 3)- xZnO glasses

Abstract

31P solid state magic angle spinning and liquid nuclear magnetic resonance spectroscopies have been used to investigate the phosphate network of (100 - x)(NaPO 3)- xZnO glasses, with 0 < × < 33.3. Addition of ZnO results in the decondensation of the sodium metaphosphate chains: the fraction of Q 2 sites decreases along with the formation of Q 1 sites. Q 1 sites corresponds to chain-end and pyrophosphate groups. The analysis of 31P chemical shifts leads to the conclusion that Zn 2+ ions are bonded only to Q 1 sites, not to Q 2 sites. Some Q 0 sites are present in the glass containing 33.3% ZnO, and are formed by the disproportionation reaction: 2Q 1 = Q 0 + Q 2. Zn 2+ coordination was evaluated from Co 2+ visible absorption spectroscopy, assuming that Co 2+ and Zn 2+ coordinations are similar. In (100 - x)(NaPO 3)- xZnO glasses, Zn 2+ is mainly octahedral whatever the x value, but the average coordination decreases as x increases. Tetrahedral Zn 2+ ions induce a higher shielding, detected by magic angle spinning nuclear magnetic resonance, on the phosphorus of the Q 1 sites to which they are bonded. The tetrahedral Zn 2+ amount increases in the glass with x = 33.3%, probably because of the formation of orthophosphate groups.