Mapping aluminum/phosphorus connectivities in aluminophosphate glasses

Two-dimensional ²⁷Al/³P cross-polarization magic-angle spinning (CPMAS) NMR experiments are performed on sodium aluminophosphate glasses in the xNaAlO₂·(1-x)NaPO₃ compositional range, providing direct detection of particular aluminum species connected to specific phosphorus species. In particular, the spectral-editing capabilities of two-dimensional experiments allow the elucidation of phosphorus resonances that are strongly correlated to four-, five and six-coordinate aluminum species. Results from two-dimensional contour plots and projections of spectra along the ³¹P dimension allow the assignment of phosphate structures that cannot be directly assigned from one-dimensional ³¹P MAS spectra. Q₁⁰, Q₃⁰ and Q₂¹ species have all been identified in these aluminophosphate glasses and tetrahedral aluminum species are bonded to each of these phosphate moieties. Changes in the phosphate/aluminate ordering are also monitored by inspection of the phosphorus subspectra of the two-dimensional CPMAS experiments, demonstrating a change in aluminum/phosphorus connectivity correlated to changes in molar composition.