CONDENSED PHONON MODES IN SUCCESSIVE PHASES OF WO3

The irreducible representations of normal modes of WO3are presented at the points Γ, X, M and R in or on the Brillouin zone of the hypothetic cubic phase. The atomic displacements at each phase are decomposed into the normal modes referring to the diffraction data. The normal modes concerned with W atoms condense at all the transition points except for the orthorhombic-monoclinic transition, where a linear combination of three rotational modes of WO6octahedra, Rx25+Rz25+My3, condenses. Antiferrodistortive structures in the phases above room temperature are due to the condensations of the M, X and R modes. Ferridistortive structure in the monoclinic (II) phase is due to the condensation of both Γz15and M´z3modes. WO3is shown to have ferridistortive structures with first-order successive phase transitions. © 1979, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.