The microstrain-accompanied structural phase transition from h-MoO₃ to α-MoO₃ investigated by in situ X-ray...

In situ X-ray diffraction indicates that the structural phase transition from h-MoO₃ to α-MoO₃ is a first-order transition with a phase transition temperature range of 378.5-443.1 °C. The linear coefficients of thermal expansion of h-MoO₃ are strongly anisotropic, that is, α₌ = 72.87 × 10^-6 K^-1 and α = -19.44 × 10^-6 K^-1. In the h-MoO₃ phase, water molecules are located at the (0 0 0.25) site inside the MoO₆ octahedra tunnel that is formed by six MoO₆ corner-sharing octahedron zigzag chains. With increasing temperature, the release of water molecules from the octahedra tunnel causes the octahedra chains to shrink and the octahedra tunnel to expand. When the phase transition occurs, the anomalous expansion of the MoO₆ octahedra tunnel ruptures the Mo-O₂ bonds, forming individual MoO₆ octahedron zigzag chains that then share corners to generate octahedron layers in the ⟨100⟩_α direction. The octahedron layers are bonded by van der Waals interactions in the ⟨010⟩_α direction, crystalizing into the α-MoO₃ structure.