Mapping aluminum/phosphorus connectivities in aluminophosphate glasses

Two-dimensional 27Al/31P 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.