Procedures for Point and Space Group Analysis

Abstract

Description of general procedures for point- and space-group determination by convergent-beam electron diffraction(CBED) soon appears in a special issue of J. Electron Microsc. Tech. The essence of them was already given in the flow charts in ref.2. We here demonstrate the procedures using an example of La2 CuO4 - 6, which is the end member of a 40K class superconductor La2-x Mx CuO4 - 6 (M = Ba, Sr and Ca). The CBED method has utilized to date only dynamical extinction(GM lines) appearing in zeroth-order Laue-zone(ZOLZ) reflections for the space-group determination. We emphasize that the use of GM lines appearing in higher-order Laue-zone(HOLZ) reflections makes it easy to determine crystal space-groups.The substance was already known to belong to the orthorhombic system with the lattice parameters of a=5.3548Å, b=5.4006Å and c=13.1592Å. Fig. 1(a) shows a CBED pattern taken at the [001] incidence, Fig.l(b) being the central part of Fig.l(a). The whole pattern has a symmetry 2mm. This indicates that the crystal has two mirror symmetries perpendicular to each other and that the point group is mmm or mm2.