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.
