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 Q^1,1 and Q^1,2 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. - Highlights: • Structure of zinc alkali pyrophosphate glasses have been analysed by 1D/2D NMR. • 2D ³¹P experiments allow to separate Q^1,1 and Q^1,2 species. • ⁶⁷Zn static NMR shows a constant signal all along the composition line