Chemistry Reference
In-Depth Information
detector in four steps as in Figure 7.3(b), the defocusing factor fluctuated around 1, with
the worst value being less than 3. By properly selecting the data collection strategy, the
defocusing effect, such as the 2u resolution, can be controlledwithin a reasonable range.
A large 2u range cylindrical detector is not recommended unless dealing with a very
small sample, similar to samples used for Debye-Scherrer cameras, such as thin wire or
powder in a capillary. If a large 2u range is required for a flat sample in reflectionmode, it
is always desirable to collect several frames at different incident (u
1
or v)anglesfor
different 2u ranges so as to improve the 2u resolution.
7.3.3 Transmission Mode Diffraction
Geometric aberrations in the transmission mode have a very different effect.
Figure 7.4 shows the geometric aberration of two-dimensional diffraction in trans-
mission mode with a thin flat sample. A parallel incident beam enters the sample
perpendicular to the sample surface, and the diffracted beams exit the sample from the
opposite surface of the sample. The beam size aberration can be expressed as
B
b
¼ cos 2uþ
t
b
sin 2u
ð7
:
17Þ
where t is the sample thickness. It can be proven that when the sample thickness is less
than 10 percent of the beam size, the defocusing factor (B/b) is equal to or less than 1
for all 2u angles. The thickness of the transmission sample is typically negligible
compared to the incident beam size (tb); therefore, ðB
bÞ¼cos 2u 1. There
should be no defocusing effect at all when the incident beam is perpendicular to
the sample surface.
Figure 7.5 is a comparison between reflection mode and transmission mode
diffraction with data frames collected from corundum powder. Figure 7.5(a) is the
data frame collected with a 0.5 mm collimator and a 5
incident angle. The diffraction
rings and the integrated profile show severe peak broadening, especially at large 2u
angles. Figure 7.5(b) is the data frame collected with the same beam size in
=
2
q
B
t
Sample
b
FIGURE 7.4 Geometry aberration of XRD
2
in transmission mode.
