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

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 monoclinic system with the space group P21 or P21/m, Z = 12, a = 17.3796(4) Å, b = 17.3847(5) Å, c = 26.1636(9) Å, β = 90.279(2)°, and V = 7905.0(3) Å3. The intermediate form crystallizes in the orthorhombic system with the space group Pbnb, Z = 12, a = 26.132(1) Å, b = 17.4206(8) Å, c = 17.3201(8) Å, and V = 7884.8(4) Å3. Both lattices shape a superstructure based on a δ-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 monoclinic cell. The high-temperature form is a good oxide-ion conductive fcc phase (a ≈ 5.6 Å) isomorphous with δ-Bi2O3. On heating, the low-temperature form transforms to the intermediate form around 40 °C with a small transition enthalpy and a minute volume contraction; the intermediate form changes to the fcc form at 780 °C with a large transition enthalpy and a noticeable volume expansion. Both transitions are reversible. The 40 °C transition is displacive and the 780 °C transition reconstructive.