Reassessment of the symmetry of the 221 PbBiSrCaCuO structure using LACBED and high-resolution SAD: the relevance of Cowley's theory of disorder scattering to a real-space structural analysis

An examination of the 221 compound of PbBiSrCaCuO using LACBED and accurately aligned high-resolution SAD patterns has led to a new assessment of its symmetry as orthorhombic in the space group Amaa, instead of monoclinic (or two-phase monoclinic and orthorhombic) as previously determined. This more accurate determination opens the way for the application of real-space methods for analysing the crystalline disorder [e.g., Cowley, Acta Cryst. A 32 (1976) 88], as an alternative to the superspace, reciprocal-space methods used in previous analyses. A brief account of the perceived gain in accuracy for both diffraction and X-ray spectroscopic analysis, resulting from the use of extended-cleavage crystals, is also given.