What is X-ray Diffraction?

What Is X-ray Diffraction?

X-ray diffraction (XRD) is a technique for examining the atomic structure of materials. During an experiment, a sample is illuminated with X-rays while the source and detector move in a synchronized way. The instrument records the signal at different angles, producing a graph with peaks that reveal information about the material’s atomic arrangement.

How Diffraction Works

• Many materials contain small crystals whose atoms are arranged in regular patterns. These patterns form distinct atomic planes separated by characteristic distances.
• X-rays have wavelengths comparable to the distances between atoms in crystals, making them useful for probing atomic-scale structure.
• When X-rays interact with electrons in atoms, they are re-emitted with the same energy. This is called elastic scattering.
• Scattered waves can reinforce or cancel one another:
  • Constructive interference occurs when waves align, amplifying the signal.
  • Destructive interference occurs when waves are out of alignment, reducing or cancelling the signal.
• Strong diffraction signals occur at particular angles where waves scattered from regularly spaced atomic planes constructively interfere. The angle between the incident and scattered beams is called 2θ.

Bragg’s Law

Constructive interference occurs when the extra distance traveled by scattered waves corresponds to a whole number of wavelengths. This relationship is expressed as Bragg’s Law:

nλ = 2d sin θ

Here, λ is the X-ray wavelength, d is the spacing between atomic planes, θ is half the angle between the incident and scattered beams, and n is an integer. Bragg’s Law connects the measured diffraction angle to the spacing between atomic planes.

The law is named for William Henry Bragg and William Lawrence Bragg, the father-and-son team who received the 1915 Nobel Prize for their work analyzing crystal structures using X-ray diffraction.

Materials and Applications

XRD can be used to study a range of materials, including single-crystal thin films, mixtures of polycrystalline powders, and amorphous materials. Applications highlighted in the video include developing pharmaceuticals, identifying minerals in rock formations, and investigating how atomic arrangements affect energy-storage materials. Advances in instrument design have also made XRD easier to use and more capable.

Speakers and Sources

• Narrator: An unnamed narrator explains X-ray diffraction and its applications.
• Source/channel: Bruker