Dear All and Dear Gerard,
I have a very little experience on Rietveld
refinement, but my professor always says to me that a negative value of
isotropic factor U has no phisical meaning, and it can be a sign that your
(fixed) occupation factors (frac) are too small; in fact frac and U-iso are
005 12:03 PMTo:
rietveld_l@ill.frSubject: Negative Thermal
parameters
I'm getting started with GSAS and
in all my refinements i get stuck with negative thermal parameters no matter
what i do. Is there any tip to get rid of this problem or maybe do i need to
learn a litt
the same experimental conditions a standard sample: Si,
quartz or LaB6, and see!
Regards
N.Masciocchi
At 18.03 31/01/05, you wrote:
I'm getting started
with GSAS and in all my refinements i get stuck with negative thermal
parameters no matter what i do. Is there any tip to get rid of this
probl
: rietveld_l@ill.fr
Subject: Negative Thermal
parameters
I'm
getting started with GSAS and in all my refinements i get stuck with negative
thermal parameters no matter what i do. Is there any tip to get rid of this
problem or maybe do i need to learn a little bit more?
Thanks
I'm getting started with GSAS and in all my refinements i
get stuck with negative thermal parameters no matter what i do. Is there any tip
to get rid of this problem or maybe do i need to learn a little bit
more?
Thanks
If you type "absorption" (or absorbtion :-) into the search field of the
Rietveld archive http://www.mail-archive.com/[EMAIL PROTECTED]/ you will
find an earlier discussion on this.
And also ~60 emails with the "absor(p/b)tion" keyword in the old
Rietveld 1994-1998 archive : http://sdpd.univ-lem
As people pointed out, thermal factors are underestimated (even negative) if
absorption is not corrected for. If you type "absorption" (or absorbtion :-) into the
search field of the Rietveld archive http://www.mail-archive.com/[EMAIL PROTECTED]/
you will find an earlier discussion on this.
Al
Sorry for the ambiguous information I gave previously. The data at
room temperature were taken with a variable wavelength (TOF) whether
at low temperature the data were recorded with constant wavelength.
There is no magnetic contribution on the pattern. The negative thermal
parameters are for the
In EXPGUI it is in the "Histogram" panel. Or, in "Expedt" go to l -
o(overall)-a(absorption). Choose function number 0 (Debye-Scherrer)...
>
>I looked quickly in GSAS manual and I colud not find the path of
>setting the absorption correction. Do you know the way of fixing these
>terms, or is ther
-
From: Maxim V. Lobanov [mailto:[EMAIL PROTECTED]
Sent: Thu 2/26/2004 1:59 PM
To: [EMAIL PROTECTED]
Cc:
Subject: RE: Re: thermal parameters at low temperature
>Sorry for the ambiguous information I gave previously. The
data
Kurt,
I mean heavy in the atomic number.
Liliana
-Original Message-
From: Kurt Leinenweber [mailto:[EMAIL PROTECTED]
Sent: Thu 2/26/2004 2:21 PM
To: [EMAIL PROTECTED]
Cc:
Subject: RE: Re: thermal parameters at low temperature
PROTECTED]
>Sorry for the ambiguous information I gave previously. The data at
>room temperature were taken with a variable wavelength (TOF) whether
>at low temperature the data were recorded with constant wavelength.
>There is no magnetic contribution on the pattern. The negative therm
atoms (in a soft portential) can get
really small ADP whereas for light atoms a considerable ADP should even be found at 0
K (Debye theory).
Andreas
> Marilena L Viciu schrieb:
>
> Dear all,
>
>
>
> I am refining a powder neutron data at low temperature (50K). I got s
t: Thu 2/26/2004 1:59 PM
To: [EMAIL PROTECTED]
Cc:
>Sorry for the ambiguous information I gave previously. The data at
>room temperature were taken with a variable wavelength (TOF) whether
>at low temperature the data were recorded with constant wavelength.
>There is no magnetic cont
>Sorry for the ambiguous information I gave previously. The data at
>room temperature were taken with a variable wavelength (TOF) whether
>at low temperature the data were recorded with constant wavelength.
>There is no magnetic contribution on the pattern. The negative thermal
>
Dear Liliana,
To guess what could be a possible answer one would probably need a little
more information.
Is the RT dataset collected with the same instrument and setup? Is it
constant-wavelength of TOF? Do you have any magnetic contribution at 50K,
and how do you treat it?
Sincerely,
correctly considered, although neutron diffraction is
less sensitive than X-ray diffraction.
Best regards
Andreas
Marilena L Viciu schrieb:
Dear all,
urn:schemas-microsoft-com:office:office" />
I am refining a powder
neutron data at low temperature (50K). I got some of the thermal pa
> ... the path of
> setting the absorption correction.
>
In expedt (after choosing the file): L, O, A, C
In EXPGUI: input window is behind "Globally Edit Absorption" buttom in the
Histogram panel.
Best Regards,
Markus
--
Markus Valkeapää, [EMAIL PROTECTED]
Inorganic Chemistry, Göteborg Univer
-
From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED]
Sent: Thu 2/26/2004 2:59 PM
To: [EMAIL PROTECTED]
Cc:
Subject: RE: Re: thermal parameters at low temperature
Liliana,
GSAS will refine the value of the
Dear all,
I am refining a powder neutron
data at low temperature (50K). I got some of the thermal parameters negative
although the model seems to be quite right (the data at room temperature behaved
well with the same model). From your experience, is this the case of a wrong
model?
I
or the ambiguous information I gave previously. The data at
>room temperature were taken with a variable wavelength (TOF) whether
>at low temperature the data were recorded with constant wavelength.
>There is no magnetic contribution on the pattern. The negative thermal
>parameters a
Title: RE: Re: thermal parameters at low temperature
ï
Liliana,
OK,
they should vibrate less... but are they good neutron scatterers or poor?
If they are poorly scattering, the thermal factors won't be as
meaningful.
- Kurt
---Kurt LeinenweberDepar
One important consideration on the issue of thermal parameters vs
occupancies is the Q-range (Q=4pisin(theta)/lambda) where useful information
exists. This is a function of the wavelength used, the resolution, the
complexity of the structure and the radiation (x-rays vs neutrons). The
best
Caroline,
This is a very thorny issue. In general, displacement [nee thermal]
parameters correlate highly with occupancy. With x-ray data, while
refinement tricks may allow both to be refined, there is not typically
enough data to allow both to be determined. With neutrons, the problem
is not as
Dear All,
I have a question regarding the correlation of thermal parameters
and occupancies. If anyone could give some tips on achieving a
satisfactory conclusion/minimum during a refinement, it would be
greatly appreciated.
At the moment I refine the atomic positions then the Us until a
>Since Matthew Henrichsen put the question to the Rietveld list two days ago
>there have been many different suggestions of possible reasons why
>"thermal" parameters are so often not reliably refined from powder X-ray
>diffraction data. I suggest that when &#x
Since Matthew Henrichsen put the question to the Rietveld list two days ago
there have been many different suggestions of possible reasons why
"thermal" parameters are so often not reliably refined from powder X-ray
diffraction data. I suggest that when 'Bragg-Brentano' geom
> Matthew Henrichsen <[EMAIL PROTECTED]> wrote 20 Jan 1999:
> >Why does x-ray powder diffraction fail? Why do the others work?
>
> If you read Rod Hill's intercomparison (J. Appl. Cryst. (1992) 25, 589-610)
> you will see that it is not only thermal parameters tha
Matthew Henrichsen <[EMAIL PROTECTED]> wrote 20 Jan 1999:
>Why does x-ray powder diffraction fail? Why do the others work?
If you read Rod Hill's intercomparison (J. Appl. Cryst. (1992) 25, 589-610)
you will see that it is not only thermal parameters that are better determi
>Why does x-ray powder diffraction fail?
Overlapping.
>Why do the others work?
No overlapping.
Overlapping leads to difficulties in locating the background
at high diffracting angle. If the background is underestimated,
the B values will be too low, if not negative, and vice versa.
Possibly,
On Mon, 19 Jan 1998 12:08:23 -0600, Matthew Henrichsen wrote:
>I've heard from others at Northwestern University using Reitveld that
>thermal parameters cannot be reliably refined from x-ray powder
>diffraction data (XRPD). There are several published reports containing
>t
I've heard from others at Northwestern University using Reitveld that
thermal parameters cannot be reliably refined from x-ray powder
diffraction data (XRPD). There are several published reports containing
thermal parameters obtained using single crystal x-ray diffraction,
diffuse
>Uii should be positive and greater than 0 to make physical sense.
Yes. In particular Uij should be a positive definite matrix. If Uii is
negative it means that some systematic error is being soaked up there
eg lack of an absorbtion correction will reduce the apparent temperature
factor, sinc
I do not know bout Lij but Uij can be negative -- However only when "i" is not equal
"j".
Uii should be positive and greater than 0 to make physical sense. Moreover,
there are other restrictions too and I can find them if you are interested.
Dr. Peter Y. ZavalijUniversity Crystallographer
Dear all,
I am working with GSAS for structure refinement. I am new in the field. So I
probably have some silly questions. When I introduce anisotropic thermal
parameters (U11, U22, ... U23) some of them are negative. Is this possible?
Should I always refine U12, U13 and U23? I used the third
Phys. Condensed Matter, 10 (1998), 3823-3832].
All the best,
Erich Kisi
>I would be interested to learn if anyone has ever published a refinement
>of anharmonic thermal parameters using powder data. Someone recently asked
>me this question, but I had no answer for them.
>
>Sincerely,
>
>Branton Campbell
I did refinement of anharmonic thermal parameters
from powder data for a few enough simple structures.
They looked very reasonable so I believe it is possible
but again only for simple enough structures.
However I never have published results simply because
I changed objects of my interest
I would be interested to learn if anyone has ever published a refinement
of anharmonic thermal parameters using powder data. Someone recently asked
me this question, but I had no answer for them.
Sincerely,
Branton Campbell
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