SYMMETRY DETERMINATION AND PB-SITE ORDERING ANALYSIS FOR THE N = 1,2,PBXBI2-XSR2CAN-1CUNO4+2N+DELTA COMPOUNDS BY CONVERGENT-BEAM AND SELECTED-AREA ELECTRON-DIFFRACTION

An examination of various preparations from the structural series PbxBi2-xSr2Can-1CunO4+2n+delta by selected-area and convergent-beam electron diffraction (SAD and CBED) shows that despite superstructural symmetries which range from monoclinic to orthorhombic, space groups Amaa or A2aa can be unambiguously identified for the subcells of different samples (here for n = 1 and n = 2 respectively), independently of the long-range superstructural result. An analysis of the n = 2 compound within the compositional range x = 0.2 --> 0.3 shows that both Pb-independent and Pb-dependent superlattices coexist for this range of x, the former superlattice retaining the superspace-group symmetry of N.A2aa.111. Following this, a semi-quantitative analysis is made using data from a commensurate Bi-Sr-Fe-O superstructural refinement. The most probable siting of Pb atoms within the Bi-O layers is determined, and a model consisting of a semi-random array of commensurate supercells derived, which accounts qualitatively for the appearances of both the observed incommensurate superlattices. A more general description of these structures requires consideration of low-angle crystallographic shear (CS) planes, identified from SAD data. Their existence may be used to explain the present variability of space group, and of superconducting properties, found for these componds over a range of cation stoichiometries and preparation procedures.