Polymorphism in Bi14WO24: A monoclinic low-temperature stable form with beta approximate to 90 degrees and its displacive transformation to an orthorhombic intermediate form at about 40 degrees C

摘要

This paper describes the preparation and phase relations between three stable polymorphs of Bi14WO24, which is the bismuth-rich neighboring phase of the high oxide-ion conductor, Bi7WO13.5. The low-temperature form crystallizes in the monoctinic system with the space group P2(1) or P2(1)/m, Z=12, a=17.3796(4) angstrom, b=17.3847(5) angstrom, c=26.1636(9) angstrom, [3=90.279(2)degrees, and V=7905.0(3) angstrom(3). The intermediate form crystallizes in the orthorhombic system with the space group Pbnb, Z=12, a=26.132(1) angstrom, b=17.4206(8) angstrom, c=17.3201(8) angstrom, and V=7884.8(4) angstrom(3). Both lattices shape a superstructure based on a delta-Bi2O3 related pseudo-fcc subcell through the transformation matrix (9/2, -3/2, 0)/(1, 3, 0)/(0, 0, 3). A slight distortion of the orthorhombic cell generates the monoctinic cell. The high-temperature form is a good oxide-ion conductive fee phase (a approximate to 5.6 angstrom) isomorphous with delta-Bi2O3. On heating, the low-temperature form transforms to the intermediate form around 40 degrees C with a small transition enthalpy and a minute volume contraction; the intermediate form changes to the fee form at 780 degrees C with a large transition enthalpy and a noticeable volume expansion. Both transitions are reversible. The 40 degrees C transition is displacive and the 780 degrees C transition reconstructive. (c) 2005 Elsevier B.V. All rights reserved.