Biomedical subjects

F N Chukhovskii

Publications and source records attributed to F N Chukhovskii.

4 recordsLinked to original sources

PubMed2003

Ab initio crystal structure determination using X-ray fluorescence holography for different noise levels: numerical simulation and analysis.

X-ray fluorescence holography (XFH) two-dimensional angular scans with the fluorescing Cu atom of a Cu(3)Au single crystal for different noise levels have been calculated and the structure factors have been numerically restored, supporting the ab initio structure determination method first discussed by Chukhovskii & Poliakov [Acta Cryst. (2003), A59, 109-116]. In the case of resultant XFH scans where noise levels are up to the regular signal values at each angular scan point, the elaborated method is found to work well. With the use of the linear regression algorithm code [Chukhovskii & Poliakov Acta Cryst. (2003), A59, 109-116], the restored structure factors show clearly not just good accuracy of the restoration code procedure but also the efficiency of the structure-determination method that can utilize the XFH data even for high noise levels.

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PubMed2003

X-ray fluorescence holography: a novel treatment for crystal structure determination.

It is shown that it is possible to use a linear regression algorithm direct method to solve crystal structures from X-ray fluorescence holography (XFH) data. It is found that, in contrast to conventional X-ray structure determination methods, which do not always work unambiguously, the sustainable method utilizing the XFH data generally provides the unique phase-retrieval structure solution and is able, in many cases, to replace the above for determining both the absolute values (moduli) and phases of structure factors. The XFH (theta, varphi) scan with a fluorescing Cu atom from a spherical cluster of a Cu(3)Au single crystal, at an energy of 10 keV for the incident unpolarized plane-wave X-radiation, is numerically simulated to test the performance of the method in finding a unique solution for the structure factors involved in the restoration procedure using the linear regression algorithm.

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PubMed2002

Domino phase-retrieval algorithm for structure determination using electron diffraction and high-resolution transmission electron microscopy patterns.

Direct-method formalism to determine atomic structures using electron diffraction data is here aimed at a general solution of the phase-retrieval problem, consequently combining electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM) patterns in a 'domino' fashion. While there are similarities to conventional (kinematical) direct methods, there remain major differences; in particular, owing to the dynamical effects in the data, the ED structure factors prove to be complex and then the positivity of the reconstructed electron density is no longer a valid constraint for 'dynamical' direct methods. Besides, owing to the dynamical effects, heavy atoms no longer dominantly contribute to the HRTEM images. Thus, the 'dynamical' direct-methods concept is based on the phase-retrieval algorithm utilizing both the dynamical ED and the HRTEM data. The fusion of the traditional direct-method technique, which is described here, allows realization of a full-phase restoration of complex structure factors. A numerical example, using the dynamical ED and HRTEM data for (Ga,In)(2)SnO(5) ceramic, shows that the method is capable of yielding a unique phase-retrieval solution. The clear sense is that the domino transform algorithm proposed works well and represents a valuable method for phasing diffraction patterns in electron structural crystallography using an experiment that is readily performed when the ED and HRTEM data are collected.

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PubMed2001

Statistical dynamical direct methods. II. The three-phase structure invariant.

The triplet distribution used for kinematical diffraction is extended to the complex case appropriate for dynamical transmission electron diffraction. It is demonstrated that this gives good results if the distributions are handled statistically rather than relying upon single triplet relationships. As a consequence, conventional statistical direct methods will yield a reasonable approximation to the effective dynamical potential for thicknesses when kinematical theory is not appropriate. The recovered effective dynamical potential may be similar to the kinematical potential, but does not have to be and in general will not be.

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