Single crystal diffraction studies of WO3 at high pressures and structure of a high-pressure WO3 phase

The structural changes of crystalline WO3 induced by the changes in pressure from ambient pressure up to 47 kbar have been studied with a diamond anvil cell. The investigations have been performed by single crystal X-ray diffraction techniques using MoK alpha radiation. The results obtained show that triclinic WO3, considered to be the thermodynamically stable form at ambient pressure, undergoes a reversible phase transition already in the range 0.3-1.2 kbar to give a monoclinic high-pressure (HP) form with a halved unit cell volume. The pressure transition, is expected as a consequence of the fact that the pseudo-C-centered lattice of the triclinic phase becomes a true lattice symmetry when the pressure is increased. Atomic displacements of only a few tenths of an Angstrom are involved in the transition. These observations agree in general with the results previously obtained by E. Salje and G. Hoppmann from their studies of the Raman spectra of WO3 at high pressure. (1980 High Temp. High Pressure 12, 213-216. The HP phase has the space group symmetry P2(1)/c, with a=5.261(1), b=5.128(1), c=7.650(4) Angstrom, and beta=92.05(3)degrees at 5.7 kbar. The compressibility of the HP phase is largest along the [010] direction. The isothermal bulk modulus, beta(0)=44.5(9) GPa, at ambient pressure and its derivative, B-0'=2.5(4), were determined by least squares techniques using the Birch equation of state. Apart from the general decrease in the cell parameters when the pressure was increased to 47 kbar, no apparent symmetry changes were detected. Thus, the monoclinic HP form appears to be the stable form in the investigated pressure range. Single crystal diffraction data of the HP form at 5.7 kbar were collected and a derived structure model was refined versus 227 reflection amplitudes to an R value of 0.040. The 3 + 3 coordination around W is more pronounced in the HP structure (three shorter bonds of 1.78 to 1.84 Angstrom and three longer bonds of 2.02 to 2.14 Angstrom) than in that at ambient pressure. The phase transition involves a significant change of the W positions relative to the centroids of the coordination octahedra. (C) 1997 Academic Press.