At 1100 K a new phase of WO3 with space group P4/ncc, a = 5.2885(5) Åand c = 7.8626(8) Å, was established, and its crystal structure was determined. The previously known tetragonal phase P4/nmm, which is shown to exist only above 1170 K, was also refined with a = 5.3031(4) Åand c = 3.9348(3) Åat 1200 K. The difference between the two perovskite-like structures relates to a tricritical non-ferroic transition characterized by a soft mode at the Z-point of the Brillouin zone, which induces antiphase rotations of the WO6 octahedra about [001]. The oxygen sublattice of the lower tetragonal structure is shown to possess the symmetry of the a0a0c- perovskite tilt structure F4/mmc (I4/mcm), which is violated by the off-centre antiferroelectric displacements of the tungsten atoms that lead to the observed space group P4/ncc.
Neutron powder diffraction results on the tetragonal-orthorhombic and orthorhombicmonoclinic structural phase transitions of tungsten oxide are reported. The observed first-order transition from P 4/ncc to P nma at 980 K to 1200 K hides the transition from the highertemperature phase P 4/nmm (via Cmca) to P nma. At 623(24) K, P nma 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.
This review summarizes recent progress in the use of X-ray diffraction for microstructural investigations. Advances in both detector architecture and computational power have led to the development of powerful yet relatively straight forward methods of analysis. Several case studies are presented.
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