Synthesis and Morphological Evolution of MoO 3 ‐II Under High Temperature and High Oxygen Partial Pressure

MoO 3 ‐II was synthesized from α‐MoO 3 at 5.5 GPa and 450°C–800°C to investigate the effects of synthesis temperature and the use of Ag 2 O as an internal oxidizing source on phase formation, surface oxygen stoichiometry, and particle evolution. X‐ray photoelectron spectroscopy (XPS) showed a higher surface oxygen stoichiometry in the Ag 2 O‐assisted samples than in their Ag 2 O‐free counterparts, except at 450°C. X‐ray diffraction (XRD) and scanning electron microscopy (SEM) further revealed distinct temperature‐dependent particle‐evolution behavior. Without Ag 2 O, both the apparent coherent‐domain size and particle size increased with synthesis temperature, whereas Ag 2 O‐assisted samples exhibited larger particles at 530°C‐600°C and finer particles at 670°C‐800°C. The contrasting trends are consistent with a possible competition between oxygen‐assisted growth and localized Ag–Mo–O reactions. These findings clarify how an internal Ag 2 O oxidizing source modifies the crystallization behavior and particle evolution of high‐pressure MoO 3 ‐II phase.