Non-homogeneous distribution of Al3+ in doped phosphate glasses revealed by 27Al/31P solid state NMR.

Solid state NMR is applied in this contribution on the xAl2O3-(50-x/2)Na2O-(50-x/2)P2O5 composition line (with 0<x<5mol%) in order to investigate the distribution of Al3+ ions in Al2O3-doped sodium phosphate glasses. The structure was analysed by (i) 27Al 1D-, 3Q-, DQ- MAS-NMR analysis and (ii) 1D 31P, 27Al(31P) 2D D-HMQC MAS-NMR and 2D 31P R-INADEQUATE technique. The 27Al NMR results confirm the presence of six-coordinated aluminate as major aluminate species and indicate that Al3+ ions are fully dissociated in the glass network. The 31P NMR data show the simultaneous presence of five different phosphate units connected to 0, 1 but also 2 Al3+ ions and offer a new vision of the doping mechanism by highlighting a non-homogeneous distribution of Al3+ ions in the phosphate matrix. This study indicates that the glass networks contain Al3+-rich and -poor domains and present thus a significant structural disorder beyond the local order.