On May 8, 2008, at 12:30 AM, May, Frank wrote:
You can check for texture effects (preferred orientation) by
obtaining multiple patterns of the material. It's realistic to
expect some differences, but preferred orientation is manifest by
not being able to replicate the pattern.
Not
Luca,
speaking about powder samples, Frank is right. The PO of powder mounts
is seldom reproducible and the filling technique is responsible for
particle orientation, depending on particle shape, filling direction,
pressure... In practice it is a nice trick to repeat the filling of the
powder
Another way to check or convince yourself of preferred orientation is to
take note what is happening when grinding the sample.
Some crystallite samples will fracture along a particular plane upon
grinding and this summed up for the whole sample may produce preferred
orientation. Alternatively,
If you have one, stick the sample on a GADDS system or similar with a 2D
detector and a large collimator. You'll find out if your sample is
grainy (or rocks in dust as Reinhard put it) in less than a minute. A
nicely micronized sample should give nice even rings with a 1mm
collimator.
Pam
Dear all,
I would like to hear opinions on the best method for the most accurate
determination of unit cell parameters from powder data. The
literature shows several techniques are used in general, e.g., Least squares
refinement of peak positions with (or without) an internal
standard, Rietveld
Forget all that long winded stuff. Just collect the data on capillary
transmission geometry and avoid all (well, most of) the fuss.
Martin Vickers
_
Be a Hero and Win with Iron Man
I do that myself but it doesn't always help much if you've got something
like wollastonite! J
From: Martin [mailto:[EMAIL PROTECTED]
Sent: May 8, 2008 10:51 AM
To: rietveld_l@ill.fr
Subject: RE: Preferred orientation?
Forget all that long winded stuff. Just collect the data on capillary
Hi all,
This thread gives me a chance to ask a question I've had for a long
time. I've heard about these large chambers where you can mix your
sample with a binder and have it fall out as small powder spheres to
avoid preferred orientation in Bragg-Brentano geometry. But, my samples
are
Kurt,
An old way used for alloys is:
grease the surface of the sample holder (preferably backgroundless) with
some sticky stuff and sieve the powder onto it. The particles will fall down
and stuck at random orientations, unless they are large plates or needles.
Peter Zavalij
Director, X-ray
Reinhard,
I stick with what Gerard said:
But i have no other information that supports the existence of
preferred orientation
so what information give you the confirmation it is the powder mount
responsible of preferred orientation. I work almost exclusively with
image plate detectors
Hairspray works quite well for thin samples (no comment from me as to
whether environmentally friendly non-aerosol is as good as aerosol as I
haven't tested it yet). Getting a nice even coverage from the sieve can
take a bit of practice as well. Together with nail varnish for sealing
capillaries
I very much agree with Luca in that graininess is not given the importance it
actually has. Older textbooks like Klug-Alexander or Peiser considered
graininess to some depth, and simple estimations show that in a usual BB
sample, the number of grains in Bragg condition may be as low as 1 for a
Dear Matt,
we have determination of unit cell parameters from powder data, using the
following software: POWDER X for treatment of data, DICVOL04 for
determination of unit cell parameters and crystalline system, NBS*AIDS83
for Least squares refinement of peak positions without internal standard.
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