BY R . SERNEELS (a), M. SNYKERS (b), P. DELAVIGNETTE (b), R. GEVERS (a, b), and S. AMELINCKX (a, b)It is shown that a strong contrast may arise in dark-field electron microscope images between two crystal parts related by an inversion operation, as a result of the violation of Friedel's law. Friedel's law is violated only in the diffracted beam and only in cases where a multiple-beam situation prevails. Moreover the excited reflections must belong to B zone which reveals the non-centrosymmetrical character of the crystal. The interfaces between two domains related by an inversion operation, which are proposed to call inversion boundaries are imaged by a-like fringes which nevertheless have properties which differ somewhat from those of pure a-fringes. These considerations are applied to the domain structure observed in the y-phase. I n this case the inversion boundaries separate regions which have suffered displacements as well. It is shown that the displacement vector of such boundaries can be given an unambiguous meaning, although strictly speaking the structures in the two parts cannot be derived one from the other by means of a parallel displacement. It is demonstrated in a concrete case that the observed contrast can be accounted for by a four. beam calculation.Es wird gezeigt, daB ein starker Kontrast auftreten kann im Dunkelfeld-Elektronenmikroskop zwischen zwei Kristallteilen, die als Ergebnis der Verletzung des Friedelschen Gesetzes durch eine Inversionsoperation verknupft sind. Das Friedelsche Gesetz wird nur im gebeugten Strahl und nur in Fallen, wo eine Mehrstrahlsituation vorherrscht, verletzt. Dariiberhinaus mussen die angeregten Reflexe zu einer Zone gehoren, die den nicht-zentrosymmetrischen Charakter des Kristalls aufzeigt. Die Grenzflachen zwischen zwei Domiinen, die durch eine Inversionsoperation verknupft sind und fur die der Name Inversions-Grenze vorgeschlagen wird, werden durch a-ahnliche Streifen abgebildet, die trotzdem Eigenschaften haben, die etwas von denen reiner a-Streifen abweichen. Diese Betrachtungen werden auf die Domanenstruktur angewendet, die in der ?-Phase beobachtet wurde. In diesem Falle trennen die Inversionsgrenzen Gebiete, die ebenfalls Verschiebungen erfahren hatten. Es wird gezeigt, daB dem Verschiebungsvektor derartiger Grenzen eine bestimmte Bedeutung gegeben werden kann, obwohl streng genommen die Strukturen in den beiden Teilen nicht von einander durch eine parallele Verschiebung abgeleitet werden konnen. I n einem konkreten Fall wird gezeigt, daD der beobachtete Kontrast durch eine Vierstrahlberechnung erkliirt werden kann.
Inversion domains are revealed in dark field images by a difference in background intensity. The inversion boundaries show an α‐like fringe contrast i.e. a symmetrical fringe pattern in bright field and an asymmetrical pattern in the dark field image. Experimental evidence for these contrast features are obtained from the non‐centrosymmetrical α‐GeTe. A theoretical analysis of the contrast characteristics is presented which is based on the logical assumption that for a non‐centro‐symmetrical crystal, the phases of extinction distance and of the absorption length are different. A very good agreement is found between theory and experiment for the contrast between the domains as well as for the fringe patterns at the boundaries. Also a particular multiple beam situation is treated.
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