Welcome Page
Course Index
Master Index
Chapter Index
Chapter 17
Aims & Objectives
This is second chapter on the subject of structure refinement
from powder diffraction data. The first section described the Rietveld method.
Now we turn to some tools that may be useful for structure refinement.
In some pages, a large amount of mathematics is given. If you don't enjoy
mathematics, please feel free to skip it. It is the end results that
are important, and not their derivation!
Specific tools that you will learn about are listed below:
-
Monitoring the molecular geometry is
one useful guide to the status of a refinement. For example, a Rietveld
refinement with the wrong space group will usually result in distortions to
the molecular geometry.
-
Fourier maps are useful tools
for the location of missing atoms in the crystal
structure. You will learn here a little on what they are and how to use
them.
-
Constraints play an essential role in
all crystallographic refinement programs. In addition,
for the Rietveld refinement of the crystal structures of organic and
organo-metallic compounds, the use of restraint functions may also be required
to overcome the limitations of powder diffraction data.
-
In practice, some data sets may be due to the diffraction of more
than one phase. So one extension of the Rietveld method has been
to multiphase refinements.
However, note that multiphase Rietveld programs should not be used as
a substitute for poor sample preparation!
-
Powder diffraction has been used extensively to study
phase transitions
in the solid state. Information obtained on one phase
can often be of use for the refinement of a related one. This page
is designed to give you a flavour of what can be done, since whole
textbooks can be (and have been) written on this subject alone.
-
Finally, this section concludes with a discussion on the simultaneous
use of two or more data sets in the Rietveld method. Rietveld refinements
using combined data sets is still
relatively rare, but can prove essential for certain solid-state problems.
Welcome Page
Course Index
Master Index
Chapter Index
