>Leonid, I
suggest you to try K4P2O7 - synchrotron data,
>space group P61 - for which
I had to put distance restraints
>on the three P2O7 independent groups -
Powder Diffraction, 28, 2-12, 2013
Done
without restraints:
http://sites.google.com/site/ddmsuite/tutorials/K4P2O7-DDM.cif
Only Biso
of chemically equivalent O-atoms are constrained, while this is not absolutely
necessary.
The DDM
plot:
http://sites.google.com/site/ddmsuite/tutorials/K4P2O7-plot.png
Characteristic
distances:
P1-O1 1.530(30) P2-O2 1.548(32) P3-O7 1.536(29)
P1-O3 1.499(26) P2-O4 1.502(23) P3-O9 1.481(31)
P1-O5 1.580(28) P2-O6 1.521(36) P3-O11 1.542(24)
P1-O13
1.614(30) P2-O14 1.576(32) P3-O14 1.719(30)
P4-O8 1.552(37) P5-O15 1.518(24) P6-O18 1.551(31)
P4-O10
1.482(22) P5-O16 1.553(37) P6-O19 1.536(23)
P4-O12
1.540(34) P5-O17 1.498(32) P6-O20 1.486(37)
P4-O13
1.689(32) P5-O21 1.600(28) P6-O21 1.674(29)
All
deviations from expected values (1.514 and 1.634) are within 3 e.s.u.s that are
around 0.03 A
in average. Further improvements seem possible, but it will require more time
and efforts.
So, one
more example of underestimated information capacity of powder data and
overestimated necessity of restraints.
*******************************************************
Leonid A. Solovyov
Institute of Chemistry and Chemical Technology
660049, K. Marx 42, Krasnoyarsk, Russia
http://sites.google.com/site/solovyovleonid
*******************************************************
________________________________
From: xtal <[email protected]>
To: [email protected]; "[email protected]" <[email protected]>
Sent: Thursday, August 1, 2013 3:10 PM
Subject: Re: [sdpd] Re: Are restraints as good as observations ?
> >Leonid, I suggest you to try K4P2O7 - synchrotron data...
>
> If you send me the data I may try.
OK.
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.
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).
BTW, try to solve the structure before the Rietveld fif.
Best,
Armel
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