Probing of Triply Coordinated Oxygen in Amorphous Al2O3

Although anomalous melt properties have been attributed to the presence of triply coordinated oxygen (O-[3]), the presence of O-[3] 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 O-17 NMR spectrum for amorphous Al2O3 and reveal the hidden nature of O-[3]. 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 O-[3] allows us to microscopically constrain its glass-forming ability and unique two-step crystallization paths of amorphous Al2O3 through the annihilation of glassy O-[3] with multiple Al-[5] species.