High-temperature phase transitions in tungsten trioxide - the last word?

The structures of tungsten trioxide, WO3, have been studied in fine temperature steps, from room temperature to 1000degreesC, by means of very high-resolution neutron powder diffraction. It was confirmed that the sample used was single-phase monoclinic in space group P2(1)/n at room temperature. In addition to this monoclinic structure, the structures observed were an orthorhombic structure in Pbcn from about 350 to 720degreesC, another monoclinic structure in P2(1)/c from about 720 to 800degreesC, a tetragonal structure in space group P4/ncc from 800 to 900degreesC, and above 900degreesC a second tetragonal structure in P4/nmm. The transformation from the Pbcn orthorhombic to the P2(1)/c monoclinic structure was certainly discontinuous, and indeed just above 720 degreesC two-phase mixtures were observed. The other transitions were continuous or nearly so, all three being apparently tricritical in nature. The sequence of phases, and nature (continuous or otherwise) of the transitions between them, can be well understood by reference to the results from a group theoretical analysis.