Phase transitions in tungsten trioxide at high temperatures - a new look

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 P 4/ ncc to Pnma at 980 K to 1200 K hides the transition from the higher-temperature phase P 4/ nmm (via Cmca ) to Pnma . At 623(24) K, Pnma transforms via octahedral rotations in a tricritical transition to P 2 1 / n . The structural characteristics and thermodynamic properties of the order parameters are described in detail. The evolution of the WO 6 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.