[sdpd] Re: Structure solution without indexing

["Peter Y. Zavalij" <zavalij...@binghamton.edu>]

I don't agree with M.Schmidt's comments simply because
"guessing, intuition or comparison with an isotypic known structure"
and
"energy minimization"
are not 
"solving a crystal structure from powder data".
It is probably applying other methods using
powder pattern simulation as good proof...


Dr. Peter Y. Zavalij  University Crystallographer
Materials Research Center, SUNY at Binghamton
Tel: (607)777-4298    Fax: (607)777-4623 
E-mail:zavalij...@binghamton.edu
http://materials.binghamton.edu/zavalij



-----Original Message-----
From: MartinUlrich_Schmidt...@de.clariant.com
[mailto:MartinUlrich_Schmidt...@de.clariant.com]
Sent: Monday, February 07, 2000 9:14 AM
To: sdpd...@egroups.com
Subject: [sdpd] Structure solution without indexing



I agree with the comments of Ken, Armel, and Robin on indexing.
Let me make just a small comment about indexing
as beeing the first step in structure determination:

There are methods for solving a structure without applying the
usual indexing procedures:
1) By guessing, intuition or comparison with an isotypic known structure
   (Many structures are solved in this way - I would assume, that more
   than 50% of all structures are either solved in this way, or never
solved)
2) By energy minimization (Prediction of possible crystal structures,
   simulation of their powder diagrams and comparison with the
   experimental powder diagram). Nevertheless, if the powder
   diagram is indexable (or even if only two lattice parameters
   can be extracted from the powder diagram), the calculation times
   diminish by a factor of at least 10-20, and the crystal structures
   can much better be found.
In this light I would say: Indexing is highly desirable (and I
always try it hardly), but from my point of view indexibility
is not a "condicio sine qua non" for solving a crystal structure
from powder data.

Martin


- - - - - - - -
Martin U. Schmidt
Clariant GmbH
Pigment Technology Research
G834
D-65926 Frankfurt a.M.
Germany
Tel.: 0049-69-305-13426
Fax: 0049-69-331749
E-Mail: MartinUlrich.Schmidt...@Clariant.com