Medium-range order in sodium phosphate glasses: A quantitative rotational echo double resonance solid state NMR study

Abstract

Sodium ultraphosphate glasses (Na₂O)_x(P₂O₅)_1−x show a strongly non-linear dependence of the glass transition temperatures T_g(x) on composition. To explore the structural origins of this behaviour, local and medium range ordering processes have been investigated by state-of-the-art ²³Na high-resolution and dipolar NMR spectroscopies. In particular, ³¹P{²³Na} and ²³Na{³¹P} rotational echo double resonance (REDOR) experiments have been analyzed to yield quantitative constraints for the structural description of these glasses. The sodium ions are found to be randomly distributed and, for x < 0.25, spatially correlated with a single metaphosphate-type Q⁽²⁾ unit at a distance of 330 pm. In this region, unusual compositional trends observed for the ²³Na chemical shifts and nuclear electric quadrupolar coupling constants, measured by triple-quantum magic-angle spinning (TQMAS) NMR, suggest a systematic decrease of Na coordination number with x. At higher sodium contents (x > 0.25), the magnitude of the ³¹P{²³Na} dipolar interaction increases markedly, indicating a significantly increased extent of Q⁽²⁾-Na-Q⁽²⁾ crosslinking. Based on these results, a comprehensive description of medium-range order in sodium ultraphosphate glasses is developed, suggesting that the T_g(x) dependence is closely linked to changes in the relative phosphorus/sodium distance distributions.