Hi Bob,
A kind of unique case isn't it ?
How many of such extreme examples can you list 18 years after it ? OK, some
papers by Margiolaki, et al...
But the question was :
>As a long time practitioner of powder XRD (since 1972) I was there when the
>superconductor craze hit in the mid-1980s. At that time it was recognized the
>Rietveld Method was useful only for REFINING structures which were generally
>accepted from single crystal x-ray analysis.
>The http://www.cristal.org/rietv025/ link says the method "[allows] an
>accurate determination of the structure." I take that statement to mean no
>model is required; Rietveld Analysis can determine structure without
>preconceived model.
i.e. a good crystallographer, when reading "determination" thinks that no model
is required. Then telling that the (Rietveld) method allows an accurate
determination of the structure is misleading since that method only allows for
a structure refinement.
So, a comment such as yours seems extremely out of the subject.
"a protein structure (a new insulin phase) was solved by rigid body Rietveld
refinement"
This is a too short and strange way to present the thing. You indexed the
pattern, guessed a model from what was already known about insulin, and had the
chance it was good enough for the Rietveld refinement convergence. The
determination did not included exclusively the Rietveld method, but also your
mind and a lot of chance- congratulations.
You dig.
Armel
> Message du 29/08/18 18:26
> De : "Von Dreele, Robert B."
> A : "Larry Finger" , "Le Bail Armel" , "Toby, Brian H."
> Copie à : "Rietveld_L@ill.fr"
> Objet : RE: Rietveld website cancelled ?
>
> Hi Armel,
> A more extreme example was in Acta D56, 1549-1533 (2000) where a protein
> structure (a new insulin phase) was solved by rigid body Rietveld refinement
> from a synchrotron powder data set & subsequently confirmed by a single
> crystal analysis (see Acta Cryst. (2001). D57, 1091-1100). NB: this kind of
> thing has also happened more recently (see papers by Margiolaki, et al).
> Bob Von Dreele
>
> -----Original Message-----
> From: rietveld_l-requ...@ill.fr On Behalf Of Larry Finger
> Sent: Wednesday, August 29, 2018 11:10 AM
> To: Le Bail Armel ; Rietveld_L@ill.fr
> Subject: Re: Rietveld website cancelled ?
>
> On 08/29/2018 04:15 AM, Le Bail Armel wrote:
> > Hi,
> >
> > After the IUCr Monograph on Crystallography 13 (2002)
> >
> > entitled "Stucture Determination from Powder Diffraction Data"
> >
> > you may find in the preface :
> >
> > "Although the Rietveld method of structure refinement from powder
> >
> > diffraction data is often loosely considered to be synonymous with
> > structure
> >
> > determination, it is not. The Rietveld method only comes into play in
> > the
> >
> > final stage of the structure solution process when an approximate
> > structural
> >
> > model has been found."
> >
> > WIF David, K Shankland, LB McCusker, C Baerlocher
> >
> > A clear distinction having my complete agreement.
> >
> > Armel
>
> Armel,
>
> Although the above statement may be generally true, I can provide a counter
> example. In Max W. Schmidt, Larry W. Finger, Ross J. Angel, and Robert E.
> Dinnabier, "Synthesis, crystal structure, and phase relations of AlSiO2OH, a
> high pressure hydrous phase", American Mineralogist, V. 83, 881-888, 1998, we
> only had a powder - no single crystals. From data measured at ESRF, we used
> GSAS to isolate the peaks of interest for the unknown phase, and used program
> TREOR to autoindex the unknown and identify the impurity phases. We then
> extracted intensities using the LeBail method, and determined the structure
> using direct methods with program SIRPOW. The structure was then refined with
> GSAS. We were even able to locate the H atom from Fourier maps!
>
> Even 20 years ago, it was possible to determine the structure from X-ray
> powder patterns.
>
> Larry
>
>
>
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