An Oxygen-Centric View of the Structure of Alkali and Alkaline-Earth Metaphosphate Glasses: Results from High-Resolution ¹⁷ O Solid State NMR Spectroscopy

The physicochemical properties of phosphate glasses are strongly influenced by the modifier cation–oxygen bonding interaction and the spatial distribution of modifier cations, yet direct experimental insight into these structural-chemical aspects remains limited. In this work, we present a comprehensive multinuclear (31P and 17O) NMR study of single- and mixed-modifier metaphosphate glasses containing alkali (Li, Na, Rb) and alkaline-earth (Mg, Ca, Ba) cations with identical nominal network connectivity. The 31P magic-angle-spinning (MAS) NMR spectra confirm that the network structure of these glasses is dominated by Q2 phosphate units, while 17O MAS and triple-quantum MAS (3QMAS) NMR provide an oxygen-centric view of bridging (BO) and nonbridging oxygen (NBO) environments and elucidate their interactions with the modifier cations. A relationship between the 17O quadrupolar asymmetry parameter and the P–O–P bond angle is established using ab initio calculations, enabling extraction of the bond-angle distribution (BAD) directly from experimental 17O 3QMAS NMR data. These distributions show increased disorder in the P–O–P BAD with increasing modifier field strength, consistent with enhanced chain folding/coiling effects. Analysis of 17O 3QMAS isotropic line shapes in single- vs mixed- modifier glasses in conjunction with bond-valence considerations indicates that the mixing between alkali and alkaline-earth cations are more uniform, while that between dissimilar alkali cations is more random. Such cation mixing behavior appears to be controlled by the structural preference to maximize a uniform spatial charge distribution and to play a key role in controlling the dependence of the glass transition and ionic transport behavior on the modifier compositional makeup.