Atomic level observations have been made for the commensurate phase P″of Fe0.902O and the incommensurate phase P′of Fe0.920O by means of the high resolution electron microscopy. The crystal structure of the P″phase is monoclinic and belongs to the space group P21/m. The vacancy cluster is made of two vacancy tetrahedra with a common edge and contains no iron atom at the tetrahedral sites. In the real crystal, however, several kinds of structural disorder are introduced. [Jap. J. Appl. Phys. 24, L723 (1985) ] . In the P′phase, the cluster arrangement is almost cubic. The P′is essentially incommensurate without any domain structure.
Profile images of surfaces of small crystals have been investigated by high resolution CTEM and computer simulation. When cubic crystals of MgO smoke with flat surfaces are oriented so that the  or  zone axis is parallel to the optical axis, two kinds of external fringes are observed at (100) surfaces ; one is parallel to the surface and the other is perpendicular to that. The results observed are compared with images simulated under considering the effects that the specimens are ionic crystals and may be charged up positively.