Thermal and storage effects on the correlation between ESR signals and optical reflectance in amorphous alumina

Amorphous alumina (Al2O3) is an essential oxide material with widespread applications in electronics, catalysis, and coatings, where its properties are critically influenced by structural disorder and defects. Unlike crystalline phases, amorphous alumina cannot be formed by melt-quenching but can be synthesized through other non-melting routes, often yielding a high fraction of AlO5 units that determine its functionality. In this study, we systematically examined how heating conditions, post-treatment atmospheres, and storage history affect defect formation and optical responses in amorphous alumina. XRD confirmed that all samples remained amorphous after heating or post-treatment. ESR and UV–Vis analyses revealed that heating at 600–800 °C generated unidentified ESR signals near g = 1.97, whereas rapid heating and quenching induced g = 2.003 signals associated with oxygen-defect-related paramagnetic centers that correlated with reduced reflectance and gray coloration. Prolonged precursor storage further enhanced these signals, highlighting the sensitivity of defect states to sample history. Post-treatments produced atmosphere-dependent defects: air and oxygen generated some unidentified ESR signals, affecting UV–visible reflectance, while ozone generated unusually strong ESR peaks at g = 2.01–2.02, resembling O3· and Al–O· radicals, which remained stable for months but primarily influenced UV absorption. Long-term storage of amorphous alumina led to the appearance of weak ESR signals and gradual increases in high-g-value signals, although reflectance remained nearly constant. Absorption spectra showed distinct peaks between 3–5 eV depending on the defect species and treatment atmosphere, emphasizing the strong coupling between defect chemistry and optical behavior. Overall, these findings demonstrate that the defect states of amorphous alumina are not fixed by intrinsic local structure alone but are governed by thermal history, treatment atmosphere, and storage conditions. The observed correlations between ESR signals and optical spectra provide fundamental insights into the defect chemistry of amorphous alumina, offering guidelines for tailoring its properties in optical, dielectric, and catalytic applications.