A complex rare-earth tungstate structure, present in a two-phased powder, was solved by electron diffraction, precession and high-resolution electron microscopy. The orthorhombic space group Pbnm and the atomic positions deduced from X-ray diffraction experiments were confirmed for Ce10W22O81. A C2/c monoclinic superstructure, with cell parameters a = 7.8, b = 36.1, c = 22.2 Å and β = 100.2°, was shown and attributed to a partial oxidation of Ce(3+) leading to interstitial oxygen ions.
Hexagonal ferrites represent an extensive family of mixed-layer magnetic materials with periods up to 1500 A along the stacking direction, probably constituting the largest unit cells in the inorganic realm. The (TS)(n)T subfamily includes P3m1 and R3m structures that can be derived from Y ferrite Ba(2)M(2)Fe(12)O(22) (M = Zn, Fe, Co, Mg, Mn) by introducing stacking faults. A unified (3 + 1)-dimensional superspace model is proposed for all members of the (TS)(n)T family. The model belongs to the superspace group X3m1(00gamma) with X = {(1/3, 2/3, 0, 1/3), (2/3, 1/3, 0, 2/3)}, has a unit cell of the basic structure with a = 5.88, c = 4.84 A and modulation vector q = 4n+3/9n+6 c*, where n is rational for periodic structures and irrational for the aperiodic ones. The model was tested on calculated data of one of the principal members of the (TS)(n)T family, the Y ferrite (n = infinity). The fit obtained with the superspace model was excellent. The model allowed a reduction of refinable parameters by 19% with respect to the ordinary refinement without a significant increase of the refinement R values.
