In situ X-ray diffraction indicates that the structural phase transition from h-MoO3to α-MoO3is a first-order transition with a phase transition temperature range of 378.5–443.1 °C. The linear coefficients of thermal expansion of h-MoO3are strongly anisotropic, that is, αa=b= 72.87 × 10−6K−1and αc= −19.44 × 10−6K−1. In the h-MoO3phase, water molecules are located at the (0 0 0.25) site inside the MoO6octahedra tunnel that is formed by six MoO6corner-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 MoO6octahedra tunnel ruptures the Mo–O2bonds, forming individual MoO6octahedron zigzag chains that then share corners to generate octahedron layers in the ⟨100⟩αdirection. The octahedron layers are bonded by van der Waals interactions in the ⟨010⟩αdirection, crystalizing into the α-MoO3structure.