Non-Monotonic Structural Response of Co3O4 during Calcination

Calcination is widely applied to adjust crystallinity and defects in metal oxides. However, structural evolution is often assumed to follow a monotonic temperature dependence. Here, we demonstrate that the structural response of Co3O4 to calcination is significantly more complex. Using a multi-modal approach combining diffraction, microscopy, spectroscopy, and defect-sensitive analysis, we track the structural evolution across multiple scales from 300–600 °C. While average crystallographic descriptors evolve monotonically with temperature, defect-associated features exhibit pronounced non-monotonic behaviour, revealing asynchronous evolution across structural levels. A distinct structural turning emerges at 400 °C, where found the metastable Co3O4. This state is characterized by enhanced local disorder and a distinct combination of various defects within spinel matrix. The results reveal that calcination does not induce a monotonic ordering pathway but activates coupled asynchronous structural processes. Our work highlights the importance of multiscale structural characterization for understanding and controlling the complex structures of oxide nanomaterials.