Wavelength and atomic dimensions
X-ray wavelengths suitable for crystallography are comparable with atomic separations, allowing waves scattered from different positions to interfere. Recorded spots or peaks are not direct photographs of individual atoms. Structural information is inferred by comparing the measured distribution with models of the arrangement and scattering process.
References: [1]
The Bragg condition
A plane-based crystal model compares the paths of waves scattered from successive planes. The Bragg relation connects wavelength, plane spacing and angle through constructive interference at whole-wavelength path differences. The angle is measured relative to the planes, requiring care when transferring conventions from other optical calculations.
References: [1]
From a pattern to material structure
Different structures yield different diffraction features that help characterize atomic arrangements and plane spacings. Interpretation also depends on specimen conditions, measurement geometry and structural assumptions. Multiple phases or imperfections can complicate the pattern, so one peak position generally does not establish every aspect of a material’s microstructure.
Establishing crystal analysis
Laue-related experiments established crystal diffraction of X-rays, and the Braggs developed structural analysis recognized by the 1915 Nobel Prize. Crystallographic methods subsequently applied interference to materials research, providing repeatable structural evidence that could be compared with theories and measurements of the physical behaviour of solids.