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

Abstract

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, αa=b = 72.87 × 10-6 K-1 and αc = -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.