Hi Ron and all,
I realize that I'm not the only one who'd like to use the Gandolfi technique
without the need to
make one's own photographic films.
In principle there are various ways to solve the problem.
One would be to mount a Gandolfi attachment on a 4-circle goniometer equipped
with a CCD
with some materials, I observed powder diffraction patterns (Bragg-Brentano,
X'pert) which
show a broad negative peak around 5-6º(2th), instead of the smoothly descending
intensity
coming from the primary beam.
Did some of you observe the same phenomenon, and has this been dealt with
anywhere?
you better refine uvw globally for the sample and then Lorentzian x and y
individually for each phase. uvw should be constant for a given
diffractometer setting and you might get it from an independent refinement
of a standard.
best
miguel
On 3 Feb 2010 at 15:41, Huy LE-QUOC wrote:
> Dear R
you also might circumvent the problem by substituting the angle with
a distance restraint (Si-Si for Si-O-Si, etc).
best
miguel
On 25 Nov 2008 at 9:39, Brian H. Toby wrote:
>
> When setting up a bond angle soft constraint in GSAS, one needs to
> input the atomic sequence numbers for the three
Dear Stephen,
years ago I wrote a program suite to determine symmetry from
observed intensities. It needs single crystal data and is not fully
automatic, i.e. it doesn't give you a list of sp gr but it calculates a
reliability index for symmetry elements and sp gr you want to check.
best
migu
Dear Mario,
I only use PO to check if it applies. If yes, I make a new preparation to avoid
PO. Also, graininess may look similar to PO, and this would surely bias QPA
if PO refinement is allowed for.
So my advice is: grind, grind, grind.
Best
miguel
On 27 Oct 2008 at 10:48, [EMAIL PROTECTE
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 Natale,
BTW, did you ever make a chemical analysis of your capillaries?
I tried once, and as far as I remember, I remained with the terrible doubt that
it was just bottle glass...
An alternative are CA (cellulose acetate) capillaries which really contain only
first row elements and can ea
Alan,
why so restrictive? In many occasions it was helpful for discussion to have a
small .gif or so. Also, I think data transfer and server capacity has increased
compared to, say, 10 y ago, and each of us receives plenty of (unrequested)
MB every day. Why should we put a priori limits to what
Dear Jim,
there was a discussion about the silicon standard a parameter some time
ago, and the surprisingly high discrepancy between the results for single
crystal and powder
543.11946(92) pm, NIST SRm640c (Si 4.9 µm)
543.101988(48) pm, Siegert & Becker 1984 (Si single crystal)
d=0.017472 = 18*su
I think that care has to be taken when defining B=noise. In many cases, B
contains some amorphous material scattering, air scattering, TDS, etc which
one might account for using a proper model. So they will scale with time
while the error part of noise will not.
miguel
On 19 Feb 2008 at 9:21,
in general, I made the observation that intensity may drop more than
expected from theory due to alignment problems. Alignment becomes
increasingly critical for smaller divergences as we normally have to sintonize
2 slits, one in the incident and one in the diffracted beam. With low
divergences
Jean-Marc,
if you don't know the structure, you will have to perform some indexing in
order to get a unit cell. If the result is not too ambiguous, the simplest way
would be to forget about the business to create peak shapes and use a
single crystal approach which allows for hkl superposition o
Mike,
I tried to make cellulose capillaries as well, but my product came out to be
quite fragile. Also, I couldn't find an easy way to fill them. Do you know the
tricks?
Miguel
On 17 Nov 2007 at 10:12, Michael Glazer wrote:
>
> There is an old method that I used to use for capillaries that
Dear Congwu,
as you put it, it definitely seems that you don't get out the MoK
radiation from your collimator. Control the alignment and make sure
the beam hits your sample.
best
miguel
On 25 Jan 2007 at 9:55, [EMAIL PROTECTED] wrote:
> Hi, Miguel, the unit is the so-called moment transfer
Congwu,
I'm getting confused with the units of your diffraction patterns, could
you just indicate 2theta which is probably the primary observation
without any assumption about lambda?
Also, I've trouble with your simulated Si pattern: there should be no
traces of Zr radiation generated peaks,
(Larry)
> Would it be possible for you to generate the data points for the same
> set of parameters with peaks at 5 and 10 degrees? That way the
> asymmetry will be really pronounced and the convolution method will be
> even more stressed. I don't expect it to make any difference, but I'd
> like
¿Sure about the cell parameter?
I was used to
Hubbard et al (1975) 543.0880(35) pm
or
Siegert & Becker (1984) 543.101988(48) pm
which already differ by 4 esd, ¿but now we would have >19 esd?
Miguel
On 3 Jul 2005 at 3:52, L. Cranswick wrote:
>
> Is there an "official" table of N
(a) graininess or (b) preferred orientation. For (b) you may try if
freeing of PO parameters improves refinement. For (a) just grind and
pack your sample again and look if this eliminates your intensity
problem.
best
mg
On 10 May 2005 at 22:05, ÿJie Peng wrote:
>
> Dear all:
> when i scan
I perfectly agree that many times lattice parameters and bond lengths
are to be given more confidence than occupation factors. It's the
canonical way to distinguish Al and Si in aluminosilicates (e.g. micas)
and we applied it successfully also to determine Li-Co exchange in
LixCoO2 battery mate
> Not sure I understand? If you have a 2D image showing powder rings
> then you should have some very good ideas about the level of
> granularity or texture in the sample. Just look for the variation in
> intensity versus azimuth? Did you mean a one dimensional image plate?
>
> One way to reduce
your data are free format with intensities as a real variable. Using
excel or any equivalent you can convert in integer intensities (e.g.
multiplying by ten to avoid errors with low values) which is the usual
data input ("counts").
Step 2 is then to convert the free 2th-I (real-integer) file in
if I remember well, Fd-3m and 5.43Å.
BTW, don't trust the flat plate sample distributed with your Philips
goniometer, it is terribly grainy (you can see it with the naked eye)
and you will never get a reasonable Rietveld fit with it!
miguel
On 2 Feb 2005 at 15:40, [EMAIL PROTECTED] wrote:
> D
R(F^2) is a "gratis" parameter which is not refined. It also depends
much on the actual Fobs extraction details which may get tricky if
there are many overlapping peaks (subgroup refinement, phase
mixtures etc).
Try to get R(F^2) from RCALC after a (GSAS) powpref-genles run
with almost zero shi
tails toward high diffraction angles may be due to stacking faults, a
typical example is the 020/110 reflection of kaolinite. For refinement
try fiddling with the stec ptec parameters in GSAS,
best
miguel
On 23 Jul 2004 at 11:46, Alan Hewat wrote:
> Posted for JM. Le-Meins whose email was rej
Dear Ling Yang,
I don't think you really have texture. What your pattern suffers from
is graininess which will, of course be modelled by some "crazy"
preferred orientation if you allow to refine it. To avoid graininess,
1- make a proper sample preparation
2- open in plane divergence, especially a
On 5 Sep 01, at 15:35, stefano agrestini wrote:
> Thank you very much Miguel,
> But it doesn't help, because my samples are ionic and the boron has
> -1 or -2 ionic state, while in the atmdata.dat file for GSAS there
> are only x-ray scattering factors values for B.
>
> Dr. Stefano Agrestini
>
>
On 5 Sep 01, at 10:43, stefano agrestini wrote:
> Dear all,
> Could anyone give me the x-ray scattering factors values for B-1 ? Is
> fundamental for the Rietveld analysis of my x-ray powder diffraction
> data.
> Thank you very much
>
> Dr. Stefano Agrestini
>
>
> "La Sapienza" University of Rome
Lachlan,
As data may be poor for many reasons, results may be unstable in
many ways. In my experience, elimination of the restraints in the
final refinement cycles gives always some change in parameters,
most time small but sometimes it happens that a homeless atom
begins to wander/vanish etc,
I would guess the trouble is coming from differences in the
scattering factor of H+, H, H- or whatever you used. But you said
that F(110) is similar in GSAS and DBWSFP?
Scattering factor, as well as SOF (as somebody proposed), could
also give an explanation for the synchrotron:conventional XRD
On 8 Nov 00, at 18:37, [EMAIL PROTECTED] wrote:
> Could someone provide me the structure for oligoclase, possibly with
> exact fraction of Ca/Na and Si/Al?
> Thanks in advance.
>Ferdinando Costantino.
>
Look at one of the excellent textbooks by
JV Smith
JV Smith & W
Dear Natale,
we use both Kodak DEF 392 and CEA Reflex 25 which give similar results. The third
emulsion actually available is Agfa Structurix D7 which gives a better resolution at
the cost of much longer exposure times, also on blue base which makes no sense for our
purpose. For digitalization, th
Dear Natale,
we use both kodak def 392 and
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