Structure refinement of commensurately modulated bismuth tungstate, Bi₂WO₆

Abstract

The displacive ferroelectric BizWO6 [Mr = 697"81, a = 5.4559 (4), b = 5.4360 (4), c = 16.4298 (17) A, Z = 4, Dx=9.512gcm -3, MoKa, A=0.7107A, #= 958-6 cm -l, F(000)= 1151.73], is described at room temperature as a commensurate modulation of an idealized Fmmm parent structure derived from an 14/mmm structure. Transmission electron microscopy clearly showed that there are coherent intergrowths of two distinct modulated variants in BizWO6 crystals. Displacive modes of inherent F2mm and Bmab symmetry are substantial and coherent over a 0108-7681/91/060870-12503.00 large volume. They reduce the space-group symmetry to B2ab. A further substantial displacive mode corresponds to rotation of corner-connected WO6 octahedra about axes parallel to e and has either of two inherent symmetries, Abam or Bbam, the difference being associated with the way this mode propagates along e. The dominant Abam mode reduces the space-group symmetry to P2~ab, while the existence of the Bbam mode reduces the intensity of h + l ---2n + 1 data and acts like a stacking fault. Group theoretical analysis of the problem details how the X-ray data can be classified so as to monitor the refinement. Anomalous dispersion selects the overall

Stability and range of the type II Bi 1−x W x O 1.5+1.5x solid solution

“…ions are coordinated to 4 and 6 oxygens in Bi14WO24 (i.e., tetrahedral environment [27,28]) and Bi2WO6 (i.e., octahedral environment [17,18]) respectively. W L3-edge XANES spectra originate predominantly from the dipole transition of the W 2p3/2 core electrons into the empty 5d states in the conduction band, which is known to be sensitive to both the W oxidation state and its local coordination environment [29,30].…”

“…The structure of the line phase Bi2WO6 is an n = 1 Aurivillius phase, composed of alternating perovskite-and fluorite-like layers [17,18].…”

Structure refinement of commensurately modulated bismuth strontium tantalate, Bi2SrTa2O9

“…A modulated-structure approach to the space group A21am as detailed by Thompson et al (1991) with our origin setting (see earlier) determines that A(h) + iA"(h) is the Fourier transform of a scattering-density component of …”

“…While these have been discussed at length elsewhere (Withers, Thompson & Rae, 1991) the key features are:…”

“…In recent years the room-temperature crystal structures of the ferroelectric Aurivillius phases, Bi4Ti30~2 (Rae, Thompson, Withers & Willis, 1990), Bi3TiNbO9 (Thompson, Rae, Withers & Craig, 1991) and Bi2WO6 (Rae, Thompson & Withers, 1991) have been re-refined from single-crystal X-ray diffraction data using a modulated structure approach. Their room-temperature structures can be described in terms of relatively small amplitude, displacive pertubations from a high-symmetry parent structure (space-group symmetry 14/mmm, ap = b e = 3.85/~, p = perovskite).…”