Re: [sdpd] Recent Congress and comiing ones about SDPD

[Armel Le Bail <alb...@cristal.org>]

>You forgot to mention the progress in indexing...? Seems like impurity
>lines are no longer a problem ;-)

Difficult to propose an impartial report about stuffs in which you are
involved. "No longer a problem" is certainly untrue.

You will find links to the powerpoint files presented about the McMaille
indexing software at Durban and Stara-Lesna at :
                                 http://www.cristal.org/new.html

It is said :
___________________________________________________________
"Provided the total intensity of the impurity lines is less than
15% of the total intensity :
- With less than 35% (in number) of impurity lines, McMaille generally
provides the correct cell in top position. However, the figures of merit
decrease.
- With 35-50% (in number) of impurity lines, McMaille may still propose
the correct cell, but generally not in first position. Thus it is more 
difficult
to locate it."

Preliminary conclusions about two-phases indexing with McMaille

Provided at least 30 lines are examined with 13-17 lines belonging to each
phases, and 40-60% of the total intensity distributed to each phase, then :
McMaille appears to be able to produce solutions in reasonable times
(<1 hour) for combinations of two phases either cubic or hexagonal or
tetragonal or orthorhombic. Monoclinic and triclinic not examined (too long).
________________________________________________________________

TREOR and ITO can cope with impurity lines as well, since many years.
Some recent changes in DICVOL allow it to propose cells that do not index
all lines. Other new indexing software claim insensitivity to impurity lines.
See the text from Robin Shirley in the CommComp Newsletter :
"Overview of powder-indexing program algorithms (history and strengths
and weaknesses" :
        http://www.iucr.org/iucr-top/comm/ccom/newsletters/2003jul/

Well, a contest about indexing powder patterns including impurity
lines could be organized. Is there a volunteer proposing 5-10 powder
patterns more or less affected ? These patterns could be synthetic
ones with arbitrary intensities, adding impurity peaks in a random way or
from a second phase, etc. Since I would like to be a participant,
I cannot organize it...

Cheers,

Armel

PS - About "realizing the obvious", some obvious things that could
be realized :
- No need to extract intensities from multipattern data for using
direct space methods, it is just necessary to be able to make
multipattern fits - for instance, FOX can produce solutions from
a neutron and an X-ray powder patterns, simultaneously. It could
be modified in order to cope with several X-ray patterns at various
temperatures, benefiting from the anisotropic thermal effects - Etc.
- Some direct space software use extracted intensities with
systems for avoiding overlapping problems (either mathematical
expression, or regenerating a pseudo powder pattern from them).
In such cases, extracting intensities from several X-ray patterns
at several temperatures and treating them with a software like
OVERLAP (excluding those hkl overlapping too much) would
produce different lists of remaining hkl. Recombinig these lists
would produce a more complete one in case of anomalous
thermal effects. No need for a simultaneous fit, recombining
individual fits would improve the hkl list as well.
- Etc


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