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 03+ 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.