Probing of Triply Coordinated Oxygen in Amorphous Al₂O₃

Although anomalous melt properties have been attributed to the presence of triply coordinated oxygen (^[3]O), the presence of ^[3]O in covalent amorphous oxides has not been revealed experimentally; such verification is the Holy Grail in the study of the physics and chemistry of glasses. We report the first ¹⁷O NMR spectrum for amorphous Al₂O₃ and reveal the hidden nature of ^[3]O. The detailed NMR characteristics of the oxygen tricluster are distinct from those estimated for the crystalline analogs, thus indicating its unique structure. This unambiguous evidence of the presence of ^[3]O allows us to microscopically constrain its glass-forming ability and unique two-step crystallization paths of amorphous Al₂O₃ through the annihilation of glassy ^[3]O with multiple ^[5]Al species.