Composition-Structure Correlations in Al2O3–NaPO3–SiO2 Mixed-Network Former Glasses: Insights from Advanced Solid-State NMR Spectroscopy

Mixed-network former glasses have drawn considerable attention for their ability to combine the advantages of different glass formers. However, chemical incompatibilities among formers often lead to phase separation, hindering practical applications. In this work, glasses with the composition xAl2O3-(80-x)NaPO3-20SiO2 (x = 0, 10, 15, 20, 25 mol %) were synthesized via the melt-quenching method. The atomic-scale structure of these glasses was elucidated using advanced solid-state nuclear magnetic resonance (SSNMR) techniques, including single pulse Magic Angle Spinning (MAS), 31P refocused Incredible Natural Abundance Double Quantum Transfer Experiment (refocused INADEQUATE), Double-Quantum based Dipolar Recoupling Effects Nuclear Alignment Reduction (DQ-DRENAR), 27Al{31P} Rotational Echo Double Resonance (REDOR), 31P{27Al} Rotational Echo Adiabatic Passage Double Resonance (REAPDOR). The results reveal that the 80NaPO3-20SiO2 glass predominantly contains homonuclear linkages (P–O–P, Si–O–Si), while heteronuclear P–O–Si bonds are less favored, leading to Na+ ions enrichment in phosphate-rich regions and evident phase separation. Incorporating Al2O3 promotes P–O–Al bond formation; when the average number of Al atoms bonded per phosphorus exceeds 2, further alumina incorporation promotes the formation of Si–O–Al linkages. Concurrently, Na+ ions begin to migrate from the vicinity of phosphate tetrahedra [PO4] toward silicate tetrahedra [SiO4]. This compositional evolution facilitates the progressive formation of Si–O–Al–O–P linkages, which effectively suppresses phase separation and yields a more homogeneous glass structure.