Dear Armel, Nita, Srebri & Carsten
May be, I am not getting the full picture but calculating cell dimensions
for isostructural compounds by only fitting the unit cell seems to me
rather useless. The only moment that I would go for something like this is
when I haven't got a clue about the structure let alone the unit cell and
space group. However, I quess we are talking also about XRD as two
different tools. First to solve and refine the structure, secondly to
follow the progress of the reaction. In both cases, if you have got some
idea what you want to make Rietveld is the way forward, even on very poor
data. We use a 20 year old Philips for the later function and a run of 30
minutes give data good enough to do a quick Rietveld. It is much faster
than doing the unit cell only as you do not have to go through the
progress of indexing your pattern, plus you do get information on the
intensities as well. Given the speed of modern computers, it would
only take about 10 minutes to get a some idea of the quality of your
sample. Once samples have passed this test, we will put it on a Siemens
D5000 with alpha1 radiation only, and do the hard work. Le Bail's method
works as well as long as your sample is highly crystalline, but it is very
good at modelling stacking errors by lowering the symmetry below that of
what you get with a Rietveld.
Furhter Armel, the reason we still study perovskites, cubic stacking and
hexagonal is that half of the universe seems to fall in this category,
well at least quite a few of the (ternary, and higher) transition metal
oxides. And just these compounds show some interesting properties, ranging
from catalysis to superconductivity. By the way new structural properties
are still discovered, see the recent work by Darriet, or charge, orbital
and spin ordering in the manganates. So still reason enough to work on
close packings!
I better get back to work than!
Best
Jaap
Jaap Vente
Inorganic Chemistry Laboratory
Department of Chemistry
University of Oxford
South Parks Road
Oxford, OX1 3QR
United Kingdom
E-mail: jaap.vente@chem.ox.ac.uk
tel: (..44)(0)1865-272602
Fax: (..44)(0)1865-272690