What is X-ray diffraction, and what is it used for?
In an X-ray diffraction experiment, a sample is placed into the center of an instrument and illuminated with a beam of X-rays.
The X-ray tube and detector move in a synchronized motion.
The signal coming from the sample is recorded and graphed,
where peaks are observed related to the atomic structure of the sample.
Most materials are made up of many small crystals like sand on a beach.
Each of these crystals is composed of a regular arrangement of atoms,
and each atom is composed of a nucleus surrounded by a cloud of electrons.
It's at this scale that the story of X-ray diffraction begins.
X-rays are high-energy light with a repeating period called the wavelength.
Since the wavelength of an X-ray is similar to the distance between atoms in a crystal,
a special interference effect called "diffraction" can be used to measure the distance between the atoms.
Interference occurs when X-rays interact with each other.
If the waves are in alignment, the signal is amplified. This is called "constructive interference".
If the waves are out of alignment, the signal is destroyed. This is called "destructive interference".
When an X-ray encounters an atom, its energy is absorbed by the electrons.
Electrons occupy special energy states around an atom. Since this is not enough energy for the electron to be released,
the energy must be re-emitted in the form of a new X-ray, but the same energy as the original.
This process is called "elastic scattering".
In a crystal, the repeating arrangement of atoms form distinct planes separated by well-defined distances.
When the atomic planes are exposed to an X-ray beam, X-rays are scattered by the regularly spaced atoms.
Strong amplification of the emitted signal occurs at very specific angles
where the scattered waves constructively interfere. This effect is called "diffraction".
The angle between the incident and the scattered beam is called 2-theta.
In order for constructive interference to occur, the scattered waves must be in alignment,
meaning that the second wave must travel a whole number of wavelengths.
In this case, one half of a wavelength is traveled on the incident side,
and one half on the scattered side, yielding one additional wavelength.
In the case of the next X-ray, one wavelength has traveled on
both the incident and the scattered side resulting in two wavelengths.
This reinforcement occurs throughout the crystal.
The exact angle at which diffraction occurs will be determined from the red triangle.
The angle at the top is theta, half the angle between the incident and scattered beams.
The long side is the distance between the atomic planes and the short side we know is one half of a wavelength.
The relationship between the diffraction angle,
and the spacing between the atoms can be determined by applying the sine function.
Rearranging this equation yields an equation commonly known as "Bragg's Law",
named after Sir William Henry and William Lawrence Bragg, the father-son team who won the Nobel Prize in 1915
for their work analyzing crystal structures with X-ray diffraction.
This technique of X-ray diffraction is used today for a wide variety of materials, ranging from
single crystal epitaxial thin films, to polycrystalline mixtures of powders, and even randomly oriented amorphous materials.
X-ray diffraction helps scientists to develop new pharmaceuticals, classify rock formations
based on their mineral components and understand how the arrangement of atoms
affects the behavior of energy storage materials.
As scientists push their ability to engineer materials on the atomic level,
X-ray diffraction becomes an increasingly important tool in their toolbox.
Advances in equipment design have made X-ray diffraction easier to use, and more powerful than ever.