X-ray diffraction (XRD) is a non-destructive technique for the qualitative and quantitative analysis of the crystalline materials, in form of powder or solid.
Basically XRD is obtained as the "reflection" of an X-ray beam from a family of parallel and equally spaced atomic planes, following the Bragg's law: when a monochromatic X-ray beam with wavelenght l is incident on lattice planes with an angle q , diffraction occurs if the path of rays reflected by successive planes (with distance d) is a multiple of the wavelenght.
l is the waveleght of the radiation coming from the X-ray tube. With a goniometer, it is possible to analyse the d spacings of a crystal (or a powder) by measuring the first-order angles of diffraction.
Qualitative analysis (phase analysis) can be done thanks to the comparison of the diffractogram obtained from the specimen with a huge number of patterns included in the official databases. Single phases and/or mixtures of phases can be analysed with the programs available today.
Many other investigations can be performed with the help of X-ray diffraction.
Residual stress: forces that results in a small compression or dilatation of the d-spacings. With XRD it is possible to measure the strain (the deformation of the original lattice) and the stress is calculated thanks to the knowledge of the elastic constants of the material.
Texture: it is the preferred orientation of the crystallites in a specimen. If a texture in a material is present, the intensity of a diffraction line changes with the orientation of the sample respect to the incident beam.
Crystallite size and microstrain: these information are obtained by the analysis of the width and the shape of the diffraction lines.
Structure analysis: XRD is used to investigate the crystallographic structure of a material. The position and the relative intensities of the diffraction lines can be correlated to the position of the atoms in the unit cell, and its dimensions. Indexing, structure refinement and simulation can be obtained with specific computer programs.
Thin film: Keeping the incident beam at low angles, it is possible to investigate the properties of multilayers, minimising the interference of the substrate. On the same way, reflectometry can be performed.