Hi to all,
@Leopoldo, thank you for providing the nice listing of the most relevant
quantities determining the reflections' positions.
However, I think there is a minor mistake in the description for the
sample displacement: The given relation (-s·cos(theta) or in full
-2s·cos(theta)/R with R = goniometer radius, e.g. Klug, Alexander 1974)
implies that it is 0 at 180°2Theta, i.e., it continuously decreases with
growing 2Theta. This also seems to be intuitive when you prepare a small
sketch on how a misplaced sample height affects a reflection's position
at low, moderate and high diffraction angles.
Best regards from Bremen,
Johannes
Am 29.03.2017 um 15:54 schrieb Cline, James Dr. (Fed):
Hi,
Seems everyone has beaten on this matter quite well. I humbly offer
this paper: http://nvlpubs.nist.gov/nistpubs/jres/120/jres.120.013.pdf
which discusses how you can experimentally determine (eliminate) the
zero error with considerable certainly.
You should consider what you know the most, and least, about: you can
never know the sample displacement or attenuation errors, you might
know the lattice parameters, and you can eliminate the zero error if
you want to. You can refine all parameters with high angle data; but
you must be careful to inspect results for loss of physical
plausibility. I do this only as an adventurous exercise.
Regards,
Jim
James P. Cline
Materials Measurement Science Division
National Institute of Standards and Technology
100 Bureau Dr. stop 8520 [ B113 / Bldg 217 ]
Gaithersburg, MD 20899-8523 USA
[email protected] <mailto:[email protected]>
(301) 975 5793
*From:*[email protected] [mailto:[email protected]]
*On Behalf Of *David L. Bish
*Sent:* Wednesday, March 29, 2017 9:41 AM
*To:* [email protected]
*Subject:* Re: using Zero
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
<[email protected]
<mailto:[email protected]>>:
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: [email protected]
<mailto:[email protected]>
[mailto:[email protected]
<mailto:[email protected]>] On Behalf Of "Lukasz
Kruszewski"
Sent: 29. mars 2017 13:25
To: Ana Isabel Becerro Nieto <[email protected]
<mailto:[email protected]>>
Cc: [email protected] <mailto:[email protected]>
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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