Powder X-Ray Diffraction
INTRODUCTION
X-rays are electromagnetic radiation of
wavelength about 1 Å (10-10 m), which is about
the same size as an atom. They occur in that
portion of the electromagnetic spectrum between
gamma-rays and the ultraviolet. The discovery
of X-rays in 1895 enabled scientists to probe
crystalline structure at the atomic level.
When certain geometric requirements are met,
X-rays scattered from a crystalline solid can
constructively interfere, producing a diffracted
beam. In 1912, W. L. Bragg recognized a
predictable relationship among several factors.
Fig. 1 Reflection of x-rays from two planes of
atoms in a solid.
The path difference between two waves:
2x = 2dsin(theta)
For constructive interference nλ = 2dsinθ
Bragg equation
X-ray diffraction has been in use in two main
areas, for:1. Fingerprint characterization of crystalline
materials and
2. The determination of their structure.
Each crystalline solid has its unique
characteristic X-ray powder pattern which
may be used as a "fingerprint" for its
identification. Once the material has been
identified, X-ray crystallography may be used
to determine its structure, i.e. how the atoms
pack together in the crystalline state and what
the interatomic distance and angle are etc. Xray diffraction is one of the most important
characterization tools used in solid state
chemistry and materials science.
We can determine the size and the shape of the
unit cell for any compound most easily using
the diffraction of x-rays.
The figure below shows the x-ray diffraction
pattern from a single crystal of a layered clay.
Strong intensities can be seen for a number of
values of n; from each of these lines we can
calculate the value of d, the interplanar spacing
between the atoms in the crystal.
Fig. 2 X-ray diffraction pattern from a layered
structure vermiculite clay.