Divergent phase evolution in mixed oxide W_0.5Mo_0.5O₃ under electron beam irradiation and thermal annealing

Mixed metal oxides such as W_0.5Mo_0.5O₃ offer a unique platform for tailoring structural and electronic properties through the synergistic integration of tungsten and molybdenum oxides, yet their phase transformation pathway is poorly understood. Here, we investigate the real-time structural evolution of W_0.5Mo_0.5O₃ using advanced in situ transmission electron microscopy (TEM). By decoupling electron-beam irradiation from thermal annealing, two distinct transformation pathways are identified. Under continuous electron irradiation at ambient temperature, orthorhombic hydrated W_0.5Mo_0.5O₃ undergoes topotactic dehydration and transforms into a hexagonal phase before complete amorphization at a critical dose of 2.45 × 10⁸ e^- nm^-2 due to radiolysis. Conversely, in situ thermal annealing up to 1200 °C facilitates a controlled reduction process. While the initial dehydration mirrors the beam-induced pathway, elevated temperatures (>600 °C) trigger the nucleation of oxygen-deficient mixed-metal oxide phase. High-resolution TEM, STEM-EDS mapping and EELS analysis confirm the formation of an orthorhombic Mo₂W₂O₁₁ framework that maintains 1 : 1 W : Mo atomic ratio. Remarkably, this phase remains crystalline and unexpectedly stable at 1200 °C, far above the reported melting range of molybdenum oxides. These findings provide a quantitative framework for understanding radiation limits in 2D-layered oxides and offer a strategic route for synthesizing thermally robust metastable phases for electrochromic and energy-storage applications.