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
grain diameter of 10 Å!
For Kurt:
try Debye Scherrer (or better Gandolfi) if you can accept the peak
broadening.
best
miguel
------- Forwarded message follows -------
Date sent: Thu, 08 May 2008 19:17:53 +0200
From: Luca Lutterotti <[EMAIL PROTECTED]>
Subject: Re: Preferred orientation?
To: [EMAIL PROTECTED]
Copies to: rietveld_l@ill.fr
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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 and I can assure you that the graininess problem
is appearing more often than the preferred orientation case. I am
working on texture mostly so I am happy when you find them, but this
case is not s frequent as people think and for sure not s frequent as
graininess.
I wait also confirmation from Gerard that his sample is a powder and
it has plate like or fiber like particles. Otherwise I will
investigate the graininess case that with a proper grinding or a
spinner is easily resolvable.
Also, for who think that because the harmonic model can fit it is for
sure preferred orientation. I can just suggest to work for a while
with real textured sample and the harmonic and see if there is really
this relationship, you may be surprise by the result.
cheers,
Luca
On May 8, 2008, at 3:08 PM, Reinhard Kleeberg wrote:
> 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 holder with different filling
> techniques to look for PO. Of course, sample graininess may be also
> a reason for not reproducible intensity, but these effects ("rocks
> in the dust") ar mostly hard to correct successfully by spherical
> harmonics as Gerard stated for his problem. In any case, the problem
> sounds to be related to sample preparation.
> Reinhard
>
> Luca Lutterotti schrieb:
>
>> 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 true,
>>
>> preferred orientation or texture are perfectly reproducible,
>> provided you use the same sample orientation. What is not
>> reproducible and probably what Frank May is referring to is not
>> preferred orientation but graininess or few big grains that do not
>> guarantee the correct statistic. So if you need to check for
>> graininess, you just move a little your sample, so the beam covers
>> a different area on the sample. If you think you have texturte, to
>> check for it you have to change the sample orientation to see a
>> change. Beware that in a Bragg-Brentano instrument turning around
>> the axis normal to the sample surface is not a valid change in
>> orientation as nothing will change for texture; you have to change
>> the sample inclination instead (omega or chi).
>>
>> Best Regards,
>>
>> Luca Lutterotti
>>
>>
>>>
>>>
>>> That's the simple test. Let us know what you find.
>>>
>>> Another issue for "improper intensities" is when the specimen is
>>> not sufficiently wide enough at low angles (typically below 20-
>>> degrees 2- Theta with copper radiation) and the x-ray beam does
>>> not fully impinge on the specimen. The observed reflections in
>>> the low angle region will be less than calculated by a modelling
>>> program.
>>>
>>> Frank May
>>> Research Investigator
>>> Department of Chemistry and Biochemistry
>>> University of Missouri - St. Louis
>>> One University Boulevard
>>> St. Louis, Missouri 63121-4499
>>>
>>> 314-516-5098
>>>
>>> ________________________________
>>>
>>> From: Gerard, Garcia S [mailto:[EMAIL PROTECTED]
>>> Sent: Wed 5/7/2008 8:57 AM
>>> To: rietveld_l@ill.fr
>>> Subject: Preferred orientation?
>>>
>>>
>>>
>>> Dear all,
>>>
>>> I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows
>>> intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to
>>> 4 Angstroms.
>>> There are overestimated peaks and also underestimated peaks.I
>>> have tried to discard factors that might cause this problem:
>>>
>>> The thermal parameters look sensible. Moreover, the data at high
>>> angle looks ok, so intensity transfer from low angle to high
>>> angle or vice versa does not seem to be the cause.
>>>
>>> Atomic positions also look sensible. And again, data at high
>>> angle looks ok. Is the scattering angle dependence of the atomic
>>> positions the same as for the thermal parameters? (I cannot
>>> remember that, but i am pretty sure it is not).
>>>
>>> Following the advice published in J. Appl. Cryst. 32, 36 (1999),
>>> the other factor that might cause this problem is preferred
>>> orientation:
>>> I have tried to find a hkl dependence in the overestimated and
>>> underestimated peaks but i could not find any. If i try to model
>>> preferred orientation with spherical harmonics the problems
>>> disappears nicely. The problem is how to justify the existence of
>>> preferred orientation. The crystal system is orthorhombic. But i
>>> have no other information that supports the existence of
>>> preferred orientation.
>>>
>>> Is there any other problem that I cannot think of?Is the
>>> preferred orientation correction masking any of these other
>>> problems I cannot think of?
>>>
>>> Regards
>>>
>>> Gerard
>>>
>>>
>>>
>>> ________________________________
>>>
>>> Heriot-Watt University is a Scottish charity registered under
>>> charity number SC000278.
>>>
>>>
>>
>
------- End of forwarded message ---------
Miguel Gregorkiewitz
Dip Scienze della Terra, Università
via Laterina 8, I-53100 Siena, Europe
fon +39'0577'233810 fax 233938
email [EMAIL PROTECTED]
