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Chapter 9
Aims & Objectives
The aim of this chapter is to follow up the basic theory given
previously with a
"nuts and bolts" explanation on how to calculate the structure factor and
intensity of a given powder reflection, in order to compare with experimental
data. This step by step approach is intended to help students, who are not
confident on all of the mathematical arguments used earlier, by going
through a specific example.
This chapter is the second of a two-part series: Diffraction Theory I
(chapter 6) and Diffraction Theory II (chapter 9) and specific
objectives are as follows:
-
To take the case of a sodium chloride sample on a laboratory powder
diffractometer as a working example. The calculations start by working out the
structure factor of the 111 reflection.
This approach is taken right the way through this chapter. First
it is explained how to do a calculation for the 111 case in preparation for a
similar calculation on a different case during the assignment later. It is
hoped that this approach will make it easier for each student, regardless of
mathematical skill, to work his/her way through the assignment.
-
Although in practice structure factor and intensity calculations are performed
by a computer it is, in the opinion of the author, useful and illuminating to
first do one such calculation on a simple case by hand.
-
After calculating the structure factor it is necessary in practice to take into
account a number of other factors which influence the experimental measurement
of the intensity. These factors are sometimes referred to as
"correction factors".
-
The final link in this whole process is to compare the calculation for the
intensity with the experimentally measured intensity. The section then
concludes with an explanation as to why the structure factor equation
itself is modified to take into account the thermal motion of the
atoms withthe crystal structure.
-
As in Diffraction Theory I, you are advised that Diffraction Theory II should
be read
sequentially and so links to all the various topics are not given here. The
student is therefore advised to start at the beginning with
Kinematic versus Dynamic Diffraction,
and work linearly through the pages to the end.
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