Short- and Medium-Range Order of Sodium Aluminoborophosphate Glasses Studied by Dipolar NMR Spectroscopy

The structure of glasses in the aluminoborophosphate glass system with compositions 40Na2O–40P2O5–(20–x)Al2O3–xB2O3 (0 ≤ x ≤ 20) has been studied by single- and double-resonance solid-state nuclear magnetic resonance (NMR) spectroscopy. The principal network forming units (NFUs) were identified and quantified by high-resolution spectra obtained by magic-angle spinning (MAS) NMR. The boron atoms are predominantly four-coordinated, while the Al species occur in four-, five- and six-fold coordination, and their average connectivity increases with increasing boron content. The connectivities between these NFUs were determined by dipolar recoupling experiments such as 11B{31P} and 27Al{31P} rotational echo double resonance (REDOR) and 31P double-quantum filtering experiments. No significant 27Al–11B interaction was detectable. The results indicate a strong preference for Al–O–P and B–O–P heteroatomic connectivities, whereas a random linkage model clearly does not provide an appropriate description. The glass transition temperature shows a characteristic nonlinear compositional dependence on x, with a maximum near x = 10. This behavior can be modeled by considering the average connectivity density of the network, calculated from the NFU distribution as deduced from the quantitative connectivity analysis. 23Na MAS NMR and 23Na{31P} REDOR results indicate that the sodium ions maintain a constant local environment dominated by the phosphate species, explainable by standard bond-valence concepts.