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Re: [sdpd] Re: Are restraints as good as observations ?
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<p>>Leonid, I<br>
suggest you to try K4P2O7 - synchrotron data,<br>
>space group P61 - for which<br>
I had to put distance restraints<br>
>on the three P2O7 independent groups -<br>
Powder Diffraction, 28, 2-12, 2013<br>
 <br>
Done<br>
without restraints:<br>
<a href="http://sites.google.com/site/ddmsuite/tutorials/K4P2O7-DDM.cif";>http://sites.google.com/site/ddmsuite/tutorials/K4P2O7-DDM.cif</a><br>
Only Biso<br>
of chemically equivalent O-atoms are constrained, while this is not absolutely<br>
necessary.<br>
The DDM<br>
plot:<br>
<a href="http://sites.google.com/site/ddmsuite/tutorials/K4P2O7-plot.png";>http://sites.google.com/site/ddmsuite/tutorials/K4P2O7-plot.png</a><br>
 <br>
Characteristic<br>
distances:<br>
P1-O1 Â 1.530(30)Â P2-O2Â 1.548(32)Â P3-O7Â 1.536(29)Â <br>
P1-O3 Â 1.499(26)Â P2-O4Â 1.502(23)Â P3-O9Â 1.481(31)Â <br>
P1-O5 Â 1.580(28)Â P2-O6Â 1.521(36)Â P3-O11 1.542(24)Â <br>
P1-O13<br>
1.614(30)Â P2-O14 1.576(32)Â P3-O14 1.719(30)Â <br>
                                    <br>
P4-O8Â 1.552(37)Â P5-O15 1.518(24)Â P6-O18 1.551(31)<br>
P4-O10<br>
1.482(22)Â P5-O16 1.553(37)Â P6-O19 1.536(23)<br>
P4-O12<br>
1.540(34)Â P5-O17 1.498(32)Â P6-O20 1.486(37)<br>
P4-O13<br>
1.689(32)Â P5-O21 1.600(28)Â P6-O21 1.674(29)<br>
 <br>
All<br>
deviations from expected values (1.514 and 1.634) are within 3 e.s.u.s that are<br>
around 0.03 A<br>
in average. Further improvements seem possible, but it will require more time<br>
and efforts.<br>
 <br>
So, one<br>
more example of underestimated information capacity of powder data and<br>
overestimated necessity of restraints.<br>
 <br>
*******************************************************<br>
Leonid A. Solovyov<br>
Institute of Chemistry and Chemical Technology<br>
660049, K. Marx 42, Krasnoyarsk, Russia<br>
<a href="http://sites.google.com/site/solovyovleonid";>http://sites.google.com/site/solovyovleonid</a><br>
*******************************************************<br>
<br>
________________________________<br>
From: xtal <<a href="mailto:xtal%40noos.fr";>xtal...@noos.fr</a>><br>
To: <a href="mailto:sdpd%40yahoogroups.com";>sdpd...@yahoogroups.com</a>; "<a href="mailto:rietveld_l%40ill.fr";>rietveld_l...@ill.fr</a>" <<a href="mailto:rietveld_l%40ill.fr";>rietveld_l...@ill.fr</a>> <br>
Sent: Thursday, August 1, 2013 3:10 PM<br>
Subject: Re: [sdpd] Re: Are restraints as good as observations ?<br>
<br>
<br>
> >Leonid, I suggest you to try K4P2O7 - synchrotron data...<br>
> <br>
> If you send me the data I may try.<br>
<br>
OK.<br>
<br>
K4P2O7 at room temperature is a good example for your horror museum of bad things done in the SDPD world and which could have been so much better if DDM was used.<br>
<br>
This case belongs to the acentric class of problems. Acentricity leads to a 2 times more unfavourable (atomic coordinates)/data ratio than for centric compounds. This is also the case for beta-Ba3AlF9 for which I can send the original 1993 data, if you wish, and you may see if Al-F distances are convincing without restraints using DDM (I obtained crazy Al-F distances in the 1.65-1.97 A range, instead of 1.81+-0.04 A for usually regular AlF6 octahedra).<br>
<br>
BTW, try to solve the structure before the Rietveld fif.<br>
<br>
Best,<br>
<br>
Armel<br>
<br>
[Non-text portions of this message have been removed]<br>
<br>
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