Abstract
Neutron powder diffraction results on the tetragonal-orthorhombic and orthorhombic-monoclinic structural phase transitions of tungsten oxide are reported. The observed first-order transition from P4/ncc to Pnma at 980 K to 1200 K hides the transition from the higher-temperature phase P4/nmm (via Cmca) to Pnma. At 623(24) K, Pnma transforms via octahedral rotations in a tricritical transition to P21/n. The structural characteristics and thermodynamic properties of the order parameters are described in detail. The evolution of the WO6 octahedra and the atomic positions is documented using such parameters as the octahedral elongation, octahedral variance and the off-centre displacement vectors for the tungsten atoms. It is shown that the phase transitions can be adequately described within the framework of a decoupled mean-field Landau theory.
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