Hi Ana Isabel,
Following up on Stan's comments, we always perform this type of
calibration with a certified standard, e.g., an NIST standard (Si or
LaB6). We then measure data from below the lowest-angle peak up the
high-angle limit of the instrument (about 155 deg. in our case). If you
use a certified standard, you can FIX the unit-cell parameter(s) and
refine zero error and specimen displacement corrections. I typically
obtain good refinements, with small esd's, when I follow this procedure.
After all, the purpose is not to obtain the unit-cell parameter of your
standard. If you have the ability to adjust the zero error of your
instrument, you can then do that after doing your refinements.
Ultimately you should be able to achieve a very small zero error.
Make sure you then use that refined zero error in subsequent
refinements, until you realign your instrument. The zero error should
not be a function of your sample. Specimen displacement is a function of
each sample (mount) and should always be refined to obtain more accurate
(not precise) unit-cell parameters.
Cheers,
Dave
On 3/29/2017 9:23 AM, Leopoldo Suescun wrote:
Hola Ana Isabel,
As Stan said, if you go high enough in 2theta you have enough data to
de-correlate zero and sample displacement corrections and you can
refine them together with lattice parameters, and have correct values
for the three.
The reason for thi si that delta(2theta) for each peak is a fixed
amount with zero (for all 2theta), proportional to -Scos(theta) with S
sample displacement and proportional to arcsin(lambda/2d) with change
of lattice parameters.
Since these three terms affecting peak positions have different
behaviors with 2theta, zero is constant, displacement is maximum at
low and high 2theta but null at 90 and lattice parameter increases the
shift with 2theta (as does the separation of Kalfa1 and Kalfa2 peaks)
then with a wide-enough patterns you will see the effects of the three
affecting differently the low, mid and high 2theta peaks, making a
refinement possible.
But again, you need to go very high in 2theta to be able to
de-correlate the three effects, collecting data from 10 to 80 degrees
won´t allow you to refine more than one of the three with confidence
(assuming the two you fix are correct).
Of course, aligning your diffractometer correctly and placing the
sample at the place where it should be is always the best choice...
Good luck with your work
Leo
2017-03-29 8:38 GMT-03:00 Julian Richard Tolchard
<julianrichard.tolch...@sintef.no
<mailto:julianrichard.tolch...@sintef.no>>:
I would also suggest that it is easier to just check that your
instrument zero is aligned than to mess around with long scans and
fitting routines to separate correlated variables.
Jools
-----Original Message-----
From: rietveld_l-requ...@ill.fr <mailto:rietveld_l-requ...@ill.fr>
[mailto:rietveld_l-requ...@ill.fr
<mailto:rietveld_l-requ...@ill.fr>] On Behalf Of "Lukasz Kruszewski"
Sent: 29. mars 2017 13:25
To: Ana Isabel Becerro Nieto <ani...@icmse.csic.es
<mailto:ani...@icmse.csic.es>>
Cc: rietveld_l@ill.fr <mailto:rietveld_l@ill.fr>
Subject: Re: using Zero
Hi. You can do that, of course, but you have to choose: you can
use Zero error OR sample displacement, but never use both (this
induces some physical impossibilities). However, I assume your
diffractometer is calibrated, and the zero position of the
detector is fine; the sample displacement may (I suppose) be
connected with preparation-derived errors, and I'd use the latter
parameter instead of the zero error.
Hope this helps anyhow. Good luck!
Luke Kruszewski
> Dear all,
>
> I am using Si as internal standard to calibrate my pattern. Should I
> refine the "Zero error" of the diffractometer if I am using the
> calibrated pattern?
>
> Thanks a lot,
>
> Ana
>
>
>
> Dra. Ana Isabel Becerro
>
> Instituto de Ciencia de Materiales de Sevilla
>
> CSIC-US
>
>
>
>
>
>
>
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