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 inter face modulated structure of a simpler basic structure, An incommensura te structure, based on the 4H structure, is attributed to a systematic ''ledging'' of the translation interfaces, similar to that causing th e occurrence of incommensurability in long-period superstructures in a lloys, This description also allows us to understand the transformatio n into the commensurate 4H structure after annealing. In situ heating inside the electron microscope transforms the 4H stacking into a disor dered 9R- or 18R-type structure, Direct imaging by high-resolution ele ctron microscopy confirms the structural models proposed for the diffe rent phases on the basis of their diffraction patterns. (C) 1998 Acade mic Press.