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

Abstract Simultaneous structural correlations between 31 P and 23 Na nuclei and between 31 P and 133 Cs 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 31 P nuclei at the center of phosphate tetrahedra and sodium (or cesium) modifier ions in the system x Cs 2 O: (56 −  x ) Na 2 O: 44 P 2 O 5 (where x  = 0 − 40) are followed systematically as a function of alkali oxide composition. The TRAPDOR method probes specific interactions between 31 P and 23 Na (or 133 Cs) nuclei through the direct nuclear spin dipolar interaction. 31 P MAS NMR spectra of these polyphosphate glasses reveal two phosphate environments corresponding to phosphorus atoms with two (Q 2 ) and one (Q 1 ) non-bridging oxygens in a 2.7:1 ratio, as expected from a simple depolymerization model. The Q 2 /Q 1 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 31 P– 23 Na pairings and a minimum interaction for 31 P– 133 Cs at concentrations of approximately 35 mol% for each of the respective alkali cations.