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 pr essure from ambient pressure up to 47 kbar have been studied with a di amond anvil cell. The investigations have been performed by single cry stal X-ray diffraction techniques using MoK alpha radiation. The resul ts obtained show that triclinic WO3, considered to be the thermodynami cally stable form at ambient pressure, undergoes a reversible phase tr ansition already in the range 0.3-1.2 kbar to give a monoclinic high-p ressure (HP) form with a halved unit cell volume. The pressure transit ion, is expected as a consequence of the fact that the pseudo-C-center ed 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 agre e in general with the results previously obtained by E. Salje and G. H oppmann from their studies of the Raman spectra of WO3 at high pressur e. (1980 High Temp. High Pressure 12, 213-216. The HP phase has the sp ace group symmetry P2(1)/c, with a=5.261(1), b=5.128(1), c=7.650(4) An gstrom, and beta=92.05(3)degrees at 5.7 kbar. The compressibility of t he 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 Bir ch equation of state. Apart from the general decrease in the cell para meters when the pressure was increased to 47 kbar, no apparent symmetr y changes were detected. Thus, the monoclinic HP form appears to be th e stable form in the investigated pressure range. Single crystal diffr action data of the HP form at 5.7 kbar were collected and a derived st ructure model was refined versus 227 reflection amplitudes to an R val ue 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 pressur e. The phase transition involves a significant change of the W positio ns relative to the centroids of the coordination octahedra. (C) 1997 A cademic Press.