Why are X-rays, not visible light, used to image crystals?

Diffraction requires a wavelength comparable to the spacing being resolved; interatomic distances are about an angstrom, which matches X-rays and thermal neutrons but is thousands of times smaller than visible-light wavelengths.

When is neutron diffraction preferred over X-rays?

Neutrons scatter from nuclei rather than electrons, so they detect light atoms like hydrogen well and are sensitive to magnetic moments, making them ideal for locating light elements and for mapping magnetic structures that X-rays largely miss.

X-ray and Neutron Diffraction

Because their wavelengths match interatomic spacings, X-rays and neutrons scatter coherently from crystal planes, and the resulting diffraction patterns reveal atomic positions in the lattice.

Definition

X-ray and neutron diffraction are techniques that determine crystal structure by measuring the directions and intensities of radiation scattered coherently from the periodic atomic array; constructive interference occurs when the Bragg or, equivalently, the Laue condition relates the scattering vector to a reciprocal lattice vector.

Scope

This topic covers the diffraction of X-rays and neutrons by crystals: the Bragg reflection law and the equivalent Laue condition, the structure and atomic form factors that set peak intensities, the Ewald-sphere construction, and the complementary information from X-ray scattering (sensitive to electron density) and neutron scattering (sensitive to nuclei and magnetic moments). It connects the reciprocal-lattice geometry of sibling topics to the experimental determination of structure, while leaving detailed instrumentation to applied fields.

Core questions

Why must the probing wavelength be comparable to interatomic spacing for diffraction to occur?
How are the Bragg reflection law and the Laue condition equivalent statements of the same physics?
What sets the intensity of a diffraction peak, and what is the structure factor?
How do X-ray and neutron scattering provide complementary information about electrons, nuclei, and spins?

Key concepts

Bragg law and Laue condition
Structure factor and atomic form factor
Ewald sphere construction
X-ray scattering from electron density
Neutron scattering from nuclei and magnetic order

Key theories

Bragg's law of diffraction: W. L. Bragg modeled diffraction as reflection from parallel lattice planes, with constructive interference when the path difference equals an integer number of wavelengths, giving the simple condition that underlies crystal structure determination.

Clinical relevance

Diffraction is the primary method for determining the atomic structure of materials and biomolecules; X-ray crystallography established the structures of DNA, proteins, and countless compounds, while neutron diffraction uniquely locates light atoms and resolves magnetic structures.

History

Von Laue's 1912 observation of X-ray diffraction from a crystal proved both the wave nature of X-rays and the lattice nature of crystals; the Braggs' formulation of the reflection law in 1913 made the method quantitative, and neutron diffraction followed once reactor sources became available in the 1940s.

Key figures

Max von Laue
William Lawrence Bragg
William Henry Bragg

Seminal works

bragg1913
ashcroft1976

Frequently asked questions

Why are X-rays, not visible light, used to image crystals?: Diffraction requires a wavelength comparable to the spacing being resolved; interatomic distances are about an angstrom, which matches X-rays and thermal neutrons but is thousands of times smaller than visible-light wavelengths.
When is neutron diffraction preferred over X-rays?: Neutrons scatter from nuclei rather than electrons, so they detect light atoms like hydrogen well and are sensitive to magnetic moments, making them ideal for locating light elements and for mapping magnetic structures that X-rays largely miss.