The High-Temperature Phases of WO3

High-temperature, high-resolution neutron powder diffraction experiments were performed to investigate the phases of WO3between room temperature and 850°C. Two phases were found and characterized by Rietveld refinements: orthorhombicWO3(Pbcn,a=7.3331(2),b=7.5733(2),c=7.7401(3) ? at 350°C, tilt systema0b+c-) and tetragonal-WO3(P4/ncc,a= 5.2759 (1),b=5.2759(1),c=7.8462(3) ? at 800°C, tilt systema0a0c-). The sequence of temperature-induced phase transitions in WO3can be rationalized in terms of changes in the octahedral tilt systems and/or displacements of the tungsten out of the center of the WO6octahedron. Above room temperature the two phase transitions are driven by sucessive softening of phonon modes,M3at the- to-transition andR25at the- to-transition.