High-temperature phase transitions in tungsten trioxide - the last word?

Abstract

The structures of tungsten trioxide, WO3, have been studied in fine temperature steps, from room temperature to 1000 °C, by means of very high-resolution neutron powder diffraction. It was confirmed that the sample used was single-phase monoclinic in space group P21/n at room temperature. In addition to this monoclinic structure, the structures observed were an orthorhombic structure in Pbcn from about 350 to 720 °C, another monoclinic structure in P21/c from about 720 to 800 °C, a tetragonal structure in space group P4/ncc from 800 to 900 °C, and above 900 °C a second tetragonal structure in P4/nmm. The transformation from the Pbcn orthorhombic to the P21/c monoclinic structure was certainly discontinuous, and indeed just above 720 °C two-phase mixtures were observed. The other transitions were continuous or nearly so, all three being apparently tricritical in nature. The sequence of phases, and nature (continuous or otherwise) of the transitions between them, can be well understood by reference to the results from a group theoretical analysis.

Electrical and optical properties of mixed phase tungsten trioxide films grown by laser pyrolysis

“…2.1 Vibrational and structural analysis Raman and XRD analysis of tungsten trioxide have been reported numerous times [22][23][24][25][26], and despite the simple stoichiometry, the structure is very complex [27][28][29]. Raman spectroscopy was done on two areas of the sample as denoted by spot 1 and spot 2 of the Raman image shown in the inset of Figure 2.…”

WO3/TiO2 nanotube photoanodes for solar water splitting with simultaneous wastewater treatment.

“…WO 3 , despite its simple stoichiometry, can be found in large variety of crystal structures (e.g. monoclinic, orthorhombic and triclinic) [22,23].…”

Structural and optical properties of visible active photocatalytic WO3 thin films prepared by reactive dc magnetron sputtering

“…WO 3 exists in several phases, which differ with respect to each other by the tilting and displacement of the atoms in the octahedral building blocks that comprise the WO 3 structure. The thermodynamically favored phase is reported to change with temperature (at standard pressure) for crystalline bulk WO 3 as follows: monoclinic Pc (e-WO 3 ) up to 230 K, triclinic P1 (d-WO 3 ) 230-300 K, monoclinic P2 1 /n (c-WO 3 ) 300-623 K, orthorhombic Pnma (b-WO 3 ) 623-1020 K, tetragonal P4/ncc (a-WO 3 ) 1020-1171 K, and, finally, tetragonal P4/nmm (a-WO 3 ) to the melting point at 1700 K. [22][23][24][25][26] The stability of each of these phases is modified by preparation procedures. For nanocrystalline thin films, the detailed growth conditions are critical, including type of substrate, temperature, and pressure.…”