Investigation of zinc alkali pyrophosphate glasses. Part II: Local and medium range orders analysed by 1D/2D NMR

The structure of the (66-x)ZnO-xNa2O-33.4P2O5 composition line, selected for the development of low-Tg and stable glasses, has been investigated by 1D/2D NMR spectroscopy. If standard 1D 31P MAS-NMR experiments give access to the Qn speciation and show the presence of Q0, Q1 and Q2 sites within the glass structure, application of the homonuclear through-space correlation technique (31P DQ-SQ) allows for a more accurate description of the phosphate units. Clear distinction between the Q1 sites involved in dimmers or in longer chains has been derived from 2D NMR correlation maps and leads to the re-assignment of Q1 into Q1,1 and Q1,2 species. 23Na and 23Na(31P) REDOR MAS-NMR experiments have been used to analyse the Na+ ions distribution and its interaction with the phosphate network. 67Zn static NMR experiments, performed at very high field, were carried out and suggest a constant Zn2+ coordination state all along the composition line. The results have been used to discuss the impact of the Zn2+/Na+ ratio on the extent of disorder within the glass network expressed in terms of Qn dismutation equilibrium constant and phosphate chain length distribution.