Solid-state NMR investigations of sodium–cesium mixed-alkali phosphate glasses

Simultaneous structural correlations between 31P and 23Na nuclei and between 31P and 133Cs nuclei in a series of mixed-alkali sodium–cesium phosphate glasses are investigated by transfer of populations via double resonance (TRAPDOR) NMR spectroscopy. The variations in spatial proximity between 31P nuclei at the center of phosphate tetrahedra and sodium (or cesium) modifier ions in the system xCs2O: (56 − x) Na2O: 44 P2O5 (where x = 0 − 40) are followed systematically as a function of alkali oxide composition. The TRAPDOR method probes specific interactions between 31P and 23Na (or 133Cs) nuclei through the direct nuclear spin dipolar interaction. 31P MAS NMR spectra of these polyphosphate glasses reveal two phosphate environments corresponding to phosphorus atoms with two (Q2) and one (Q1) non-bridging oxygens in a 2.7:1 ratio, as expected from a simple depolymerization model. The Q2/Q1 ratios measured using the TRAPDOR technique show a mixed-alkali effect as a function of alkali oxide concentration, revealing a maximum in the effect for 31P–23Na pairings and a minimum interaction for 31P–133Cs at concentrations of approximately 35 mol% for each of the respective alkali cations.