[Ifeffit] Modelling texture in Artemis

Matt Newville newville at cars.uchicago.edu
Wed Aug 15 09:10:35 CDT 2018

Hi Kristine,

On Tue, Aug 14, 2018 at 7:09 AM Kirstine Dalgaard <kjd at inano.au.dk> wrote:

> Dear all
> How can texture in a powder sample be modeled in ARTEMIS?
I am not sure that it can be.  EXAFS is sensitive to local structure around
the absorbing atom and is not sensitive to crystallinity or long range
order.  Well, EXAFS *is* sensitive to the relative orientation of the local
structure to the polarization of the incident X-ray beam (usually
horizontal unless you're purposely changing the polarization, but since you
didn't mention this, I'll assume you are not doing this).

My samples are fine powders of a layered material, mixed with cellulose and
> pressed to pellets. The data were collected in fluorescence mode with a 45
> degree tilt.
> PXRD data reveal that the samples are highly textured, including
> standards.

My understanding is that texture in powder diffraction reflects a limited
range of orientations in a sample, insofar as there may be preferential
ordering and an incomplete distribution of orientations.   But crystal
orientation and local structure are not necessarily the same thing.  If the
texturing means some interatomic bonds are preferentially oriented with
respect to the horizontal plane then maybe there would be an effect.  But
that more or less requires the local structure (not the crystal structure)
to be anisotropic around the absorbing atom.    That is, even if the
crystal is a layered material, if the absorbing atom has an isotropic
environment, there will likely be very little or no effect.

On the other hand, if the structure is layered and those layers are
preferentially in the horizontal plane, then EXAFS will be preferentially
probing bonds along the layer planes.

Sorry if that sounds too vague, but I cannot give a more specific answer
without knowing more about the system.  Maybe someone else here can.

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