Electron microscopy of a family of hexagonal perovskites:: One-dimensional structures related to Sr4Ni3O9

The structure of the compound Sr4Ni3O9 can be described as a stacking of close-packed SrO3 layers with omission of oxygens and insertion of nickel ions. The 4H structure can be considered as a translation interface modulated structure of a simpler basic structure, An incommensurate structure, based on the 4H structure, is attributed to a systematic "ledging" of the translation interfaces, similar to that causing the occurrence of incommensurability in long-period superstructures in alloys, This description also allows us to understand the transformation into the commensurate 4H structure after annealing. In situ heating inside the electron microscope transforms the 4H stacking into a disordered 9R- or 18R-type structure, Direct imaging by high-resolution electron microscopy confirms the structural models proposed for the different phases on the basis of their diffraction patterns. (C) 1998 Academic Press.