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[sdpd] (333) reflection for MgAl2O4 ?

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      <p>Dear All:
<br>

<br>
I didn't find anyone giving a datum for (333) reflection of MgAl2O4 in the PDF database. 
<br>

<br>
What's the reason? I don't find any reason for it to be a destructive interference.
<br>

<br>
Thanks a lot.
<br>
ike
<br>

<br>

<br>

<br>

<br>

<br>
å??件人ï¼? Armel Le Bail 
<br>
å??é??æ?¶é?´ï¼? 2011-10-20  15:02:01 
<br>
�件人� sdpd 
<br>
æ??é??ï¼? 
<br>
主�� Re: [sdpd] Re: DDM program update 1.92 
<br>
 
<br>
  
<br>

<br>
&gt; &gt; maybe you could also explain why it is normally not possible to
<br>
&gt; &gt; refine positions of H atoms from single crystal data...
<br>

<br>
I can't find who said that ? Not me.
<br>

<br>
&gt;I also have to apply constraints in some cases, but I hate them and always
<br>
&gt;try to be unconstrained as much as possible.
<br>

<br>
Fully agree with you.
<br>

<br>
Is it not another way to say that H atoms are a plague and a curse 
<br>
(full original
<br>
text below) - due to the need to constrain them in a majority of 
<br>
Powder X-ray studies ?
<br>

<br>
Best,
<br>

<br>
Armel
<br>

<br>
&gt;Hi,
<br>
&gt;
<br>
&gt;A recurrent subject... Hydrogen atoms are a kind of curse to
<br>
&gt;conventional X-ray Powder Diffraction SDPD experts.
<br>
&gt;
<br>
&gt;No kidding, a real plague.
<br>
&gt;
<br>
&gt;With or without them, in general you can obtain a good fit.
<br>
&gt;
<br>
&gt;You rarely can see them on a Fourier difference map.
<br>
&gt;
<br>
&gt;If you try to publish without them, invariably a referee requires firmly
<br>
&gt;that you make more efforts.
<br>
&gt;
<br>
&gt;If you publish with them, they will be generally guessed at highly
<br>
&gt;probable positions by many software, eventually optimized, but when
<br>
&gt;inserted into the Rietveld refinement, even at fixed positions, then
<br>
&gt;the positions of the other atoms will move a bit, and their distances
<br>
&gt;with the H atoms will change if compared with your optimization.
<br>
&gt;Then you may have to introduce constraints or restaints or rigid body
<br>
&gt;and so on, then you may want to go back to your optimization,
<br>
&gt;then back to the Rietveld fit, then ....
<br>
&gt;
<br>
&gt;A curse...
<br>
&gt;
<br>
&gt;But a complete hydrogen bonding scheme is so nice.
<br>
&gt;Anyway, you cannot discuss it too much since the H atoms were
<br>
&gt;guessed and more or less fixed, etc.
<br>
&gt;
<br>
&gt;A plague...
<br>
&gt;
<br>
&gt;If one proposes a one-button to press process incorporated into
<br>
&gt;a Rietveld software, this would be quite nice (Lachlan would agree
<br>
&gt;probably with that query to the developers).
<br>
&gt;
<br>
&gt;Best wishes,
<br>
&gt;
<br>
&gt;Armel
<br>

<br>

<br>

<br>
<br>
[Non-text portions of this message have been removed]<br>
<br>
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