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Probing Compositional Order in Atomic Columns: STEM Simulations Beyond the Virtual Crystal Approximation

Published online by Cambridge University Press:  16 December 2019

Douglas A. Blom  and
Thomas Vogt
Douglas A. Blom*
Affiliation:
Department of Chemical Engineering and NanoCenter, University of South Carolina, 715 Sumter St., Room 001, Columbia, SC29208, USA
Thomas Vogt
Affiliation:
Department of Chemistry and Biochemistry and NanoCenter, University of South Carolina, 631 Sumter St., Columbia, SC29208, USA
*
*Author for correspondence: Douglas A. Blom, E-mail: doug.blom@sc.edu
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Abstract

Taking advantage of recent advances in parallel computing, we studied compositional disorder along metal–oxygen atomic columns in a complex Mo,V-oxide bronze using multislice frozen-phonon calculations. Commonly, the virtual crystal approximation (VCA) is used to model compositional disorder at crystallographic sites in a unit cell for a number of different theoretical and experimental techniques. In the VCA, a weighted linear sum of atomic properties is used to approximate the model structure. When using the VCA, the extracted V content of Mo,V–O columns from experimental high-angle annular dark-field (HAADF) images will be about half the V content estimated from simulations, considering the distinct cation ordering. This discrepancy is larger than the spread of HAADF signals of different configurational orders at a given V concentration, which can be up to 20%. Certain “isophilic” atomic arrangements along the column can be distinguished from more random ones using HAADF-STEM imaging. The trends and ratios of the simulated intensity spreads due to different compositional ordering along 11 M–O columns along the c-axis of the Mo,V oxide bronze qualitatively match those observed in experimental HAADF-STEM data. Instrumental and sample-based noise adds to the variability but does not significantly distort the relative ratios of column intensity variation. We observed that we only required seven random configurations to represent the intensity variations along columns.

Keywords

compositional disorderHAADF-STEM imaginginstrumental and sample-based noiseMo,V-bronzemultislice frozen-phonon simulationsvirtual crystal approximation
Type
Materials Science Applications
Information
Microscopy and Microanalysis , Volume 26 , Issue 1 , February 2020 , pp. 46 - 52
Copyright
Copyright © Microscopy Society of America 2019

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