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Periodic Cation Segregation in Cs0.44[Nb2.54W2.46O14] Quantified by High-Resolution Scanning Transmission Electron Microscopy

Published online by Cambridge University Press:  01 July 2014

Markus Heidelmann ,
Juri Barthel ,
Gerhard Cox  and
Thomas E. Weirich
Markus Heidelmann*
Affiliation:
Central Facility for Electron Microscopy, RWTH Aachen University, 52074 Aachen, Germany Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
Juri Barthel
Affiliation:
Central Facility for Electron Microscopy, RWTH Aachen University, 52074 Aachen, Germany Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
Gerhard Cox
Affiliation:
BASF SE, Department of Polymer Physics, 67065 Ludwigshafen, Germany
Thomas E. Weirich
Affiliation:
Central Facility for Electron Microscopy, RWTH Aachen University, 52074 Aachen, Germany Institute of Crystallography, RWTH Aachen University, 52056 Aachen, Germany
*
*Corresponding author. [email protected]
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Abstract

The atomic structure of Cs0.44[Nb2.54W2.46O14] closely resembles the structure of the most active catalyst for the synthesis of acrylic acid, the M1 phase of ${\rm Mo}_{{{\rm 10}}} {\rm V}_{{\rm 2}}^{{{\rm 4{\plus}}}} {\rm Nb}_{2} {\rm TeO}_{{{\rm 42}{\minus}x}} $ . Consistently with observations made for the latter compound, the high-angle electron scattering signal recorded by scanning transmission electron microscopy shows a significant intensity variation, which repeats periodically with the projected crystallographic unit cell. The occupation factors for the individual mixed Nb/W atomic columns are extracted from the observed intensity variations. For this purpose, experimental images and simulated images are compared on an identical intensity scale, which enables a quantification of the cation distribution. According to our analysis specific sites possess low tungsten concentrations of 25%, whereas other sites have tungsten concentrations above 70%. These findings allow us to refine the existing structure model of the target compound, which has until now described a uniform distribution of the niobium and tungsten atoms in the unit cell, showing that the similarity between Cs0.44[Nb2.54W2.46O14] and the related catalytic compounds also extends to the level of the cation segregation.

Keywords

HRSTEMmultislice calculationatomic column compositioncation segregation
Type
Materials Applications
Information
Microscopy and Microanalysis , Volume 20 , Issue 5 , October 2014 , pp. 1453 - 1462
Copyright
© Microscopy Society of America 2014 

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Footnotes

✠

Dedicated to the memory of Dr. Gerhard Cox, who passed away on February 17, 2014.

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