Chemistry Reference
In-Depth Information
Fig. 2 Schematic diagram
showing the sequence of
stages involved in crystal
structure determination from
powder XRD data
Unit Cell
Determination
Profile Fitting
(Le Bail or Pawley Methods)
Space Group
Assignment
Structure Solution
Structure Refinement
Rietveld Refinement
2.4 Unit Cell Determination (Indexing)
The first stage of structure determination involves determination of the unit cell
{
a
,
b
,
c
,
} by analysis of the peak positions in the powder XRD pattern [this
task is referred to as “indexing” the powder pattern, and involves assigning the
Miller indices (
h
,
k
,
l
) to each observed peak in the experimental powder XRD
pattern]. Clearly, the structure determination process can progress to the structure
solution and structure refinement stages
only
if the correct unit cell is found at
the indexing stage, and difficulties encountered in achieving reliable indexing of
powder XRD patterns can sometimes be an insurmountable hurdle that prevents
successful structure determination.
The different strategies that are available for indexing powder XRD data (which
are incorporated into the widely used programs ITO [
24
], TREOR [
25
], DICVOL
[
26
] and CRYSFIRE [
29
]) generally consider the measured positions of peak
maxima for about 20 selected peaks at low diffraction angles. The existence of
significant peak overlap can be particularly problematic in the indexing stage, as
certain peaks that may be vital for correct indexing may be obscured or completely
unresolved due to peak overlap. In this regard, successful indexing of powder XRD
data in the case of molecular solids can sometimes be particularly challenging, as a
consequence of the substantial peak overlap often observed for such materials, as
noted above.
a
,
b
,
g
2.5 Preparing the Intensity Data for Structure
Solution: Profile Fitting
After the unit cell has been determined, the next stage is to prepare the intensity data
for space group determination and structure solution using an appropriate profile
fitting technique such as the Pawley method [
30
] or the Le Bail method [
31
].
The aim of this stage of the structure determination process is to fit the complete
experimental powder XRD profile by refinement of variables that describe:
