Cation sites in mixed-alkali phosphate glasses

Two lithium/sodium mixed-alkali phosphate glasses were investigated with solid-state two-dimensional heteronuclear correlation NMR. The systems represent two distinct families of mixed-alkali glasses, as they contain a fixed sodium-to-lithium ratio of 1:3, but different overall concentrations of cations. In both glasses, direct NMR evidence is found for the presence of multiple sodium cation environments using frequency-selective editing of two-dimensional correlation spectra. Frequency-specific subspectra were obtained by projection of ²³Na resonances correlated to distinct phosphate tetrahedral environments. Dynamic-angle-spinning (DAS) NMR provides for high-resolution subspectra of quadrupolar ²³Na nuclei allowing spectral separation of multiple distinct sodium environments. Identification of distinct cation sites is important for understanding the physical origin of the 'mixed-alkali effect', where non-linear behavior of physical properties is observed in glasses as a function of relative cation populations.