Electron microscopy studies of the long-range ordering in α, β and γ phases of BiLaxSr3−xO5.5−y

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Selected area electron diffraction and high-resolution transmission electron 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 β (x=0), α (x=0.2) and γ (x=0.833). Their superstructure unit cells were determined in relation to the basic perovskite lattice and the superstructure unit cell parameters were estimated basing on the X-ray powder diffraction data. For β-phase: a=20.03 Å, b=30.63 Å, c=17.90 Å, α=90°, β=90°, γ=100.22°. For α-phase: a=12.08 Å, b=16.63 Å, c=12.13 Å, α=90°, β=90.18°, γ=90°. For γ-phase: a=14.67 Å, b=10.39 Å, c=11.98 Å, α=89.82°, β=89.73°, γ=90.20°. It is supposed that the superstructure of the β-phase is a slightly deformed rhombohedral stacking of the basic perovskite blocks. The superstructure unit cell of the α-phase is a monoclinic modification of the multiplied perovskite cell. The γ-phase exhibits triclinic deformation of the perovskite lattice but for some crystallites a complex displacively modulated or even composite-type structure was observed. All samples appeared to be more or less sensitive to the focused electron beam.