X-ray diffraction (XRD) uses the diffraction pattern from a crystalline specimen to infer lattice spacings, crystal structure and phase content.

In a common powder experiment, many small crystallites are present with different orientations. For each family of lattice planes, some crystallites satisfy Bragg's condition as the scattering angle is scanned. The instrument records intensity versus $2\theta$.

Peak positions and indexing

A measured peak position gives a plane spacing through Bragg's law

$$2d\sin\theta=n\lambda.$$

Assigning a diffraction peak to a crystallographic plane family is called indexing. Its Miller indices $(hkl)$ identify which planes produced that reflection. The consistent set of indexed $d$ spacings acts as a structural fingerprint, so comparing several peaks with predicted or reference patterns can identify crystalline phases more reliably than using one peak alone.

Intensities and widths

Peak intensities depend on which atoms occupy the unit cell and on experimental geometry. Peak broadening can also contain information about small coherent domain size, lattice strain and instrument resolution. These effects must be separated before assigning a physical cause to width.

What XRD does not see equally well

An amorphous material lacks long-range periodic order, so it produces broad diffuse scattering rather than the sharp Bragg peaks of a well-crystallized phase. Minor phases can also fall below detection limits or have overlapping peaks.

XRD therefore complements microscopy: microscopy shows where structures occur locally, while diffraction measures periodic crystal information averaged over the illuminated specimen volume.