HIGH-RESOLUTION ELECTRON-MICROSCOPIC STUDY OF BA7SC6AL2O19 AND RELATED PHASES

It is shown that the crystal structures of the compound Ba7 Sc6Al2O19 and of related structures which belong to the class of hexagonal perov skites can be interpreted as mixed-layer structures consisting of the intergrowth of structural blocks (or lamellae) of beta-Ba2ScAlO5 and B a3Sc4O9, two other hexagonal perovskites. The block sequences have bee n imaged directly by means of high-resolution electron microscopy. The characteristics geometrical features of the structural microscopy. Th e characteristic geometrical features of the structural blocks allow u nambiguous identification of the different sequences. The image interp retation was confirmed by computer-simulated images. The structures ca n alternatively be considered as interface modulated structures derive d from a cubic stacking of close-packed BaO3 layers in which periodic stacking faults and twin boundaries are introduced. These interfaces a re stabilized by B cations. (C) 1994 Academic Press, Inc.