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Understanding Powder X‑ray Diffraction Profiles from Layered Minerals: The Case of Kaolinite Nanocrystals
journal contribution
posted on 2020-04-01, 15:37 authored by Alberto Leonardi, David L. BishPowder
X-ray diffraction (PXRD) techniques are widely used to characterize
the nature of stacking of submicrometer-wide nanometer-thick layers
that form layered mineral nanocrystals, but application of these methods
to infer the in-plane configuration of the layers is difficult. Line-profile-analysis
algorithms based on the Bragg equation cannot describe the broken
periodicity in the stacking direction. The Debye scattering equation
is an alternative approach, but it is limited by the large-scale atomistic
models required to capture the multiscale nature of the layered systems.
Here, we solve the Debye scattering equation for kaolinite nanocrystals
to understand the contribution of different layer-stacking defects
to PXRD profiles. We chose kaolinite as a case study because its approximately
constant composition and lack of interlayer expansion ensure that
interstitial cations and/or molecules and substitutional ions can
be ignored. We investigated the structure factor change as a function
of crystal structural and microstructural features such as layer structure
in-plane misorientation and shift (in or out of the 2D plane) and
the diameter, number, and lateral indentation of the layers. Perfect
and turbostratic stacking configurations bounded the range of intensity
variation for hkl and 00l reflections,
as well as for any scattering angle. A unique degree of disorder was
computed by the average deviation from such limiting cases, and multivariate
analysis was used to interpret the observed diffraction profiles.
Analysis of the data for KGa-1, KGa-2, and API-9 standard kaolinites
demonstrated that the estimated densities of different stacking defects
are highly correlated. In addition, analysis of API-9 particle-size
fractions revealed a dispersion of four or more components in the
standard sample. The results illustrate that the use of a distribution
of sizes, defects, and even individual kaolinite components is necessary
to accurately characterize any sample of natural kaolinite.
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Keywords
interlayer expansionkaolinite nanocrystalssubmicrometer-wide nanometer-thick layerscase studyPXRD profileslayer-stacking defectsBragg equationAPI -92 D plane00 l reflectionsdiffraction profilesin-plane configurationsubstitutional ionsatomistic modelsintensity variationLayered Mineralsmineral nanocrystalsstructure factor changemultiscale naturemultivariate analysisAPI -9 particle-size fractionsKaolinite Nanocrystals Powder X-ray diffractionsamplealternative approachDebyemicrostructural featureskaolinite componentsKGalayer structure in-plane misorientationLine-profile-analysis algorithms