Comment on ‘‘Accurate structure-factor phase determination by electron diffraction in noncentrosymmetric crystals’’

Bird, David M.; James, D.; King, Quentin A.

doi:10.1103/physrevlett.63.1118

Cited by 9 publications

(4 citation statements)

References 3 publications

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“…Our analysis of the influence of a non-zero A~p¢ in non-centrosymmetric crystals has concentrated on two-beam theory. To go beyond this in any general way is very difficult- Snykers et al (1977) and Bird, James & King (1989) discuss particular examples of the effect in three-beam diffraction. It is for this reason in § 2 that we concentrated on the change which A¢c induces in the basic Fourier coefficients.…”

Section: Discussionmentioning

confidence: 99%

“…The new work presented here is a discussion of those effects which are specific to non-centrosymmetric crystals. In this case, the Fourier coefficients Vg and Vg of the elastic and absorptive potentials repectively are both complex numbers and there is no reason in principle why the phases, ~pg and ~pg, of these coefficients should be the same (Gevers, Blank & Amelinckx, 1966;Snykers, Serneels, Delavignette, Gevers, Van Landuyt & Amelinckx, 1977;Bird, James & King, 1989;. However, in the vast majority of diffraction calculations it is assumed that the absorptive potential is directly proportional to the elastic potential, in which case the phase difference, A~pg = ~pg-~pg, vanishes.…”

Section: Introductionmentioning

confidence: 99%

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Absorption in high-energy electron diffraction from non-centrosymmetric crystals

Bird¹

1990

Acta Crystallogr A Found Crystallogr

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Absorption in high-energy electron diffraction from non-centrosymmetric crystals

Bird¹

1990

Acta Crystallogr A Found Crystallogr

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“…One example of this is for non-centrosymmetric crystals where both Vg and Vg have an amplitude and a phase. The 10% rule assumes that the phases of the two are the same, but there is no reason in principle why this should be the case, and a phase difference can lead to interesting and observable effects (Gevers, Blank & Amelinckx, 1966;Bird, James & King, 1989;Bird, 1990). With electron microscopy becoming increasingly quantitative, both in high-resolution imaging (e.g.…”

Section: )Vgmentioning

confidence: 99%

Absorptive form factors for high-energy electron diffraction

Bird¹,

King²

1990

Acta Cryst Sect A

257

102

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The thermal diffuse scattering contribution to the absorptive potential in high-energy electron diffraction is calculated in the form of an absorptive contribution to the atomic form factor. To do this, the Einstein model of lattice vibrations is used, with isotropic Debye-Waller factors. The absorptive form factors are calculated as a function of scattering vector s and temperature factor M on a grid which enables polynomial interpolation of the results to be accurate to better than 2% for much of the ranges 0-< Ms 2<-6 and 0_< M_<2/~2. The computed values, together with an interpolation routine, have been incorporated into a Fortran subroutine which calculates both the real and absorptive form factors for 54 atomic species.

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“…9(a), it was possible to obtain both the 002 and 004 Bragg conditions in the central disc, and this condition was used for refinement of the U (200) shown), in which both the 002 and 002 reflections were obtained at their Bragg conditions in a single CBED recording. Here, use is made of the fact that the difference in the intensity of the g and -g reflections in an acentric crystal is sensitive to the difference between the phases of the structure factors of the elastic and absorption potentials (Bird, James & King, 1989). The data in Fig.…”

Section: F/ Lmentioning

confidence: 99%

On the accurate measurement of structure-factor amplitudes and phases by electron diffraction

Spence¹

1993

Acta Cryst Sect A

138

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A review is given of research into the measurement of crystal structure-factor amplitudes and phases by transmission electron diffraction. Accuracies for amplitudes are commonly a fraction of a percent (after conversion to X-ray structure factors) while phases may now be measured in favorable cases using three-beam electron diffraction to an accuracy of much better than 1 °. Following a brief review of theory, the main techniques are outlined. These include quantitative convergent-beam electron diffraction, the critical-voltage method, the intersecting K_ikuchi-and HOLZ-Iine methods and methods based on weak high-order reflections in wide-angle patterns. Enantiomorphs and polarity are discussed. Summaries are given of measurements of the mean inner potential and of structure factors generally. A brief review of the implications of this work for studies of crystal bonding and ordering in alloys is given, and its use to test ab initio computations of charge density. The mean inner potential is found to be the quantity most sensitive to the bonding distribution of valence electrons and sensitively constrains accurate X-ray structure-factor measurements. Electron diffraction techniques are to be preferred for studies of individual microcrystals or artificially formed structures and for the very accurate measurement of structure-factor phases in acentric crystals with small unit cells.

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Comment on ‘‘Accurate structure-factor phase determination by electron diffraction in noncentrosymmetric crystals’’

Cited by 9 publications

References 3 publications

Absorption in high-energy electron diffraction from non-centrosymmetric crystals

Absorption in high-energy electron diffraction from non-centrosymmetric crystals

Absorptive form factors for high-energy electron diffraction

On the accurate measurement of structure-factor amplitudes and phases by electron diffraction

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