CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

53
eliminated by use of standard which is generally a single crystal and has peak near that
particular 2 value. The broadening is calculated as given below.
(2.6)
2 2 2
D I
Here
D
arises from domain size and
L
arises from instrumental factors. As mentioned
earlier this is a crude method for estimating size as broadening can also arise from stress in
the sample. Therefore D
is also a function of stress and samples are assumed to be spherical.
2.4. 2. Instrumentation
Figure 2.1 shows a schematic of XRD instrument. The first step is creation X-Rays
which is accomplished by bombarding high energy electrons on a metal target. By this
process, apart from broad distribution of X-rays created due to deceleration on electrons also
called Bremsstrahlen, sharp lines are also created which are superimposed on the broad
distribution. These sharp lines arise from characteristic X-ray coming from knocking down
of core electrons creating a “hole” and subsequent filling of higher electrons. Performing a
diffraction experiment requires intense sharp X-ray as the diffraction angle is also dependent
on x-ray energies; therefore a broad distribution is ineffective for performing diffraction. The
mainly used targets in a set up are metals like copper, molybdenum, chromium etc. The
filtering of broad distribution is accomplished by using a filter which has a 1 unit atomic
number less than the target material. The resultant beam is further monochromatised by using
a single crystal monochromator. In a PXRD, the samples in powdered form are spread over
an amorphous substrate (generally glass slides) in presence of a binder like colloidion in
amyl alcohol. The detectors are generally scintillation counters. The detector is scanned over