The microstrain-accompanied structural phase transition from h-MoO(3) to α-MoO(3) investigated by in situ X-ray diffraction

In situ X-ray diffraction indicates that the structural phase transition from h-MoO(3) to α-MoO(3) 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-MoO(3) are strongly anisotropic, that is, α(a)(=)(b) = 72.87 × 10(−6) K(−1) and α(c) = −19.44 × 10(−6) K(−1). In the h-MoO(3) phase, water molecules are located at the (0 0 0.25) site inside the MoO(6) octahedra tunnel that is formed by six MoO(6) 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 MoO(6) octahedra tunnel ruptures the Mo–O(2) bonds, forming individual MoO(6) 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-MoO(3) structure.