Electron microscopy studies of the long-range ordering in alpha, beta and gamma phases of BiLaxSr3-O-x(5.5-y)

Selected area electron diffraction and high-resolution transmission electro n microscopy together with the crystallographic image processing procedure allowed us to characterize the superstructures of all three phases existing along the BiSr3-xLaxO5.5-y line of the Bi-Sr-La-O system and denoted as be ta (x=0), a (x=0.2) and gamma (x=0.833). Their superstructure unit cells we re determined in relation to the basic perovskite lattice and the superstru cture unit cell parameters were estimated basing on the X-ray powder diffra ction data. For beta-phase: a=20.03 Angstrom, b=30.63 Angstrom, c=17.90 Ang strom, alpha=90 degrees, beta=90 degrees, gamma=100.22 degrees. For a-phase : a=12.08 A, b=16.63 A, c=12.13 A, a=90", beta=90.18 degrees, gamma=90 degr ees. For gamma-phase: a=14.67 Angstrom, b=10.39 Angstrom, c=11.98 A, alpha= 89.82 degrees, beta=89.73 degrees, gamma=90.20 degrees. It is supposed that the superstructure of the beta-phase is a slightly deformed rhombohedral s tacking of the basic perovskite blocks. The superstructure unit cell of the alpha-phase is a monoclinic modification of the multiplied perovskite cell . The gamma-phase exhibits triclinic deformation of the perovskite lattice but for some crystallites a complex displacively modulated or even composit e-type structure was observed. All samples appeared to be more or less sens itive to the focused electron beam. (C) 1999 Elsevier Science S.A. All righ ts reserved.