What is X-ray diffraction?

The physics of diffraction

In a crystal, atoms are arranged in a regular, repeating lattice, forming sets of parallel planes separated by distances similar to the wavelength of X-rays. When an X-ray beam strikes the crystal, each plane scatters part of it. In most directions the scattered waves cancel out, but in particular directions they are in step and reinforce one another — constructive interference — producing a strong diffracted beam. The directions in which this happens depend on the spacing between the planes, so measuring the diffraction pattern allows that spacing, and hence the atomic arrangement, to be deduced.

Bragg’s law and crystallography

The condition for constructive interference is described by Bragg’s law (nλ = 2d sin θ), which relates the X-ray wavelength λ, the spacing d between crystal planes, and the angle θ at which a diffracted beam appears. William Henry Bragg and William Lawrence Bragg formulated it, sharing the 1915 Nobel Prize in Physics.

From the full set of diffraction angles and intensities, crystallographers reconstruct the positions of atoms in the unit cell. Applied to protein crystallography, the same principle has solved the three-dimensional structures of countless biological macromolecules.

Uses in research

X-ray diffraction is the principal method for determining crystal structures in chemistry, physics, geology, and structural biology. It identifies minerals and crystalline phases, measures lattice parameters, and reveals the atomic and molecular architecture of solids. For very large molecules it complements imaging methods such as cryo-electron microscopy. Reliable structure determination depends on high-quality crystals and on depositing the resulting data and models in shared, citable structural databases.