The high-temperature phases of WO3

High-temperature, high-resolution neutron powder diffraction experiments were performed to investigate the phases of WO3 between room temperature and 850 degrees C. Two phases were found and characterized by Rietveld refinements: orthorhombic beta-WO3 (Pbcn, a = 7.3331(2), b = 7.5733(2), c = 7.7401(3) Angstrom at 350 degrees C, tilt system a(0)b(+)c(-)) and tetragonal alpha-WO3 (P4/ncc, a = 5.2759(1), b = 5.2759(1), c = 7.8462(3) Angstrom at 800 degrees C, tilt system a(0)a(0)c(-)). The sequence of temperature-induced phase transitions in WO3 can be rationalized in terms of changes in the octahedral tilt systems and/or displacements of the tungsten out of the center of the WO6 octahedron. Above room temperature the two phase transitions are driven by sucessive softening of phonon modes, Mj at the alpha- to beta-transition and R-25 at the beta- to gamma-transition. (C) 1999 Academic Press.