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Systematic intensity errors and model imperfection as the consequence of spectral truncation

Systematic intensity errors and model imperfection as the consequence of spectral truncation The wavelength dispersion in a graphite (002) monochromated Mo Kα beam was analyzed. A wavelength window was found with  Å, i.e.. The very large dispersion leads to systematic errors in caused by scan‐angle‐induced spectral truncation. A limit on the scan angle during data collection is unavoidable, in order that an ω2θ measurement should not encompass neighboring reflections. The systematic intensity errors increase with the Bragg angle. Therefore they influence the refined X‐ray structure by adding a truncational component to the temperature factor: B(X‐ray) = B(true) + B(truncation). For an Mo tube at 50 kV, we find B(truncation) = 0.05 Å2, whereas a value of 0.22 Å2 applies to the same tube but operated at 25 kV. The values of B(truncation) are temperature independent. The model bias was verified via a series of experimental data collections on spherical crystals of nickel sulfate hexahydrate and ammonium hydrogen tartrate. Monochromatic reference structures were obtained via a synchrotron experiment and via a `balanced' tube experiment. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Acta Crystallographica Section A: Foundations and Advances Wiley

Systematic intensity errors and model imperfection as the consequence of spectral truncation

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Publisher
Wiley
Copyright
Copyright © 2000 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0108-7673
eISSN
1600-5724
DOI
10.1107/S0108767300002853
Publisher site
See Article on Publisher Site

Abstract

The wavelength dispersion in a graphite (002) monochromated Mo Kα beam was analyzed. A wavelength window was found with  Å, i.e.. The very large dispersion leads to systematic errors in caused by scan‐angle‐induced spectral truncation. A limit on the scan angle during data collection is unavoidable, in order that an ω2θ measurement should not encompass neighboring reflections. The systematic intensity errors increase with the Bragg angle. Therefore they influence the refined X‐ray structure by adding a truncational component to the temperature factor: B(X‐ray) = B(true) + B(truncation). For an Mo tube at 50 kV, we find B(truncation) = 0.05 Å2, whereas a value of 0.22 Å2 applies to the same tube but operated at 25 kV. The values of B(truncation) are temperature independent. The model bias was verified via a series of experimental data collections on spherical crystals of nickel sulfate hexahydrate and ammonium hydrogen tartrate. Monochromatic reference structures were obtained via a synchrotron experiment and via a `balanced' tube experiment.

Journal

Acta Crystallographica Section A: Foundations and AdvancesWiley

Published: May 1, 2000

Keywords: ; ;

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