X-RAY DIFFRACTION

About 95% of all solids can be described as crystalline.

An electron in an alternating electromagnetic field will oscillate with the same frequency as the field. When an X-ray beam hits an atom, the electrons around the atom start to oscillate with the same frequency as the incoming beam. In almost all directions we will have destructive interference, that is, the combining waves are out of phase and there is no resultant energy leaving the solid sample. However the atoms in a crystal are arranged in a regular pattern, and in a very few directions we will have constructive interference. The waves will be in phase and there will be well defined X-ray beams leaving the sample at various directions. Hence, a diffracted beam may be described as a beam composed of a large number of scattered rays mutually reinforcing one another.

X-rays: 0.1 – 100 oA.
For analytical purposes 0.7 – 2 oA is the most useful region.

This technique deals with the removal of electrons from the inner shells of atom.

If K-shell loses its electron and its replaced by electron from L-shell, the resulting X-ray is called k X-ray and its energy is given as:
EK = EL - EK

Electrons in the atom emit the X-ray at same frequency as that incident ray in all directions. If the waves undergo constructive interference and are said to be diffracted by crystal plane.
Each crystalline stance diffracts the radiation in own unique way and it is governed by Bragg’s equation.
n(lamda)=2d sin(theta)

APPLICATIONS:

1. Structure of crystals: - size of crystal planes can be measured.
2. Polymer characterization: Powder method can be used for this. Non-crystalline portion scatters the radiation and crystalline portion diffracts it.
3. Polymer crystallinity : A polymer can be considered partly crystalline and partly amorphous. The crystalline domains act as a reinforcing grid, like the iron framework in concrete, and improves the performance over a wide range of temperature. However, too much crystallinity causes brittleness. The crystallinity parts give sharp narrow diffraction peaks and the amorphous component gives a very broad peak (halo). The ratio between these intensities can be used to calculate the amount of crystallinity in the material.
4. Particle size determination:
a. Spot counting method: used for determining particle size less than 5 microns. The pattern consists of series of lines or rings which appear like spots.