A new method is proposed to estimate the defocus (Af) from a single electron micrograph (EM). The method has been tested by simulations using theoretical EM's calculated under different defocus conditions. The preliminary method is successful except when the EM is taken near the optimum defocus. This can be improved by making use of the information from the electron diffraction pattern. The method will be effective for radiation-sensitive materials.
Certain space groups often permit the generation of pairs of triple relationships involving the same three parent reflections in different symmetry forms, giving rise to two equally probable invariant estimates which, because of the space-group symmetry, must disagree by an a priori known phase shift. The 230 space groups have been examined to identify those which permit inconsistent triples, and the complete list which describes the forms of the pair of triples and their phase inconsistency is given.
E. MICHALSKI, S. KACZMAREK AND M. DEMIANIUK 657 total disorder (continuous diffuse lines instead of reflexions on X-ray diffraction photographs).Moreover, we must pay attention to the fact that the different types of faults exert a similar influence on different points of the reciprocal lattice. Thus it is not possible to distinguish between some types of faults on the basis of the above parameters. One could try also to find expressions for measurable parameters describing lattice-point asymmetry and changes in the integrated intensity, as was done by Prasad & Lele (1971). However, these changes and peak asymmetry are usually too small to be estimated with sufficient accuracy. Thus peak shifts and half widths are recognized to be the best measures of faultiness. This was shown by Pandey & Krishna (1976) for the 6H(33) structure.The limitations of our theory and inaccuracy in the results which follow from the assumption of small values of OLjk are the next problem for discussion. We will show that this assumption does not limit the generality of the above theory because only small values of OLjk have physical sense. In order to jtistify the above statement let us recall the definition of probability ajk. It is equal to the ratio of the number of layers followed by faults of a particular type to the full number of layers in the examined sequence. For example, in the following sequence of an 8H (44) we have a(33) = 4/80 = 0.05. It is clear that consideration of these faults as the (33) type in 8H(44) structures makes sense only for o~(33)< 0" 1. For a~33)> 0" 1 the frequency of the occurrence of faults of (33) type is so great that the Zhdanov symbols (33) must be united in groups and it is necessary to interpret this sequence as a 6H(33) structure with stacking faults of (4) type. For example, it is necessary to interpret the sequence (33433433433334) as a 6H(33) structure with o~(4 ) = 4/46 but not as an 8H(44) structure with tx(33)=5/46. We expect that on X-ray diffraction photographs from the structure with this sequence the peak maxima will occur near the positions corresponding to those for a 6H(33) structure.The assumption of a random distribution of single faults does not limit our theory either. In general, when this assumption is not fulfilled another polytypic structure is formed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.