FERROELECTRIC TUNGSTEN TRIOXIDE

The structure of a low-temperature form of tungsten trioxide, epsilon- WO3, has been determined for the first time. Earlier inconclusive evid ence for ferroelectricity in this form is confirmed. There are at leas t six forms of WO3, but epsilon-WO3 is the only form now shown to be a centric. The structure analysis of epsilon-WO3 was based primarily on Rietveld analysis of high-resolution neutron diffraction data obtained at 15 K. Higher resolution synchrotron X-ray diffraction data were us ed to confirm the space group. This epsilon-form of WO3 is monoclinic with a space group of Pc, a = 5.278 Angstrom, b = 5.156 Angstrom, c = 7.664 Angstrom, beta = 91.762 degrees, and Z = 4. During the transform ation from triclinic delta-WO3 to the monoclinic epsilon-WO3, no chang e in the octahedral tilt system occurs, but there are significant shif ts in the W atom positions. These W atom shifts result in an enhanceme nt of the long-short bond distance alternation in the [110] direction (the [100] direction of the delta-phase), which is related to changes in electrical and optical properties. (C) 1997 Academic Press.