Transition to a new tetragonal phase of WO₃: crystal structure and distortion parameters

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.