X-ray diffraction (XRD) uses X-rays to investigate and quantify the crystalline nature of materials by measuring the diffraction of X-rays from the planes of atoms within the material. It is sensitive to both the type of and relative position of atoms in the material as well as the length scale over which the crystalline order persists. It can, therefore, be used to measure the crystalline content of materials; identify the crystalline phases present (including the quantification of mixtures in favourable cases); determine the spacing between lattice planes and the length scales over which they persist; and to study preferential ordering and epitaxial growth of crystallites. In essence it probes length scales from approximately sub angstroms to a few nm and is sensitive to ordering over tens of nanometres.

Different disciplines sometimes have different names for this technique:

• Materials science : XRD (X-ray diffraction)
• Chemistry : PXRD (powder XRD)
• Physics : WAXS (wide angle X-ray scattering)

Samples analysed using XRD are typically in the form of finely divided powders, but diffraction can also be obtained from surfaces and bulk specimens. Applications of XRD include:

Pharmaceutical industry

XRD is the key technique for solid-state drug analysis, benefiting all stages of drug development, testing and production. Many pharmaceuticals are crystalline in nature, and XRD is used to unambiguously characterize them. For example, once an active drug has been isolated, an indexed X-ray powder diffraction pattern is used to identify the crystal structure, secure a patent and protect the company’s investment. For multi-component formulations, the percentages of the active ingredients in the final dosage form can be accurately quantified, along with the percentage of any amorphous packing ingredients used.

Forensic science

XRD is used mainly in contact trace analysis. Examples of contact traces are paint flakes, hair, glass fragments, stains of any description and loose powdered materials. Identification and comparison of trace quantities of material can help in the conviction or exoneration of a person suspected of involvement in a crime.

Geological applications

XRD is the key tool in mineral exploration. Mineralogists have been amongst the foremost to develop and exploit the field of X-ray crystallography after its discovery. Thus, the advent of XRD has literally revolutionized the geological sciences to such a degree that they have become unthinkable without this tool. Nowadays, any geological group actively involved in mineralogical studies would be lost without XRD to unambiguously characterise the individual crystal structures within a specimen. Each mineral type is defined by a characteristic crystal structure, which will give a unique X-ray diffraction pattern, allowing rapid identification of minerals present within a mineral sample. The XRD data can be quantified to determine the proportion of the different minerals present.

Glass industry

While glasses are X-ray amorphous and do not themselves give X-ray diffraction patterns, there are still manifold uses of XRD in the glass industry. They include identification of crystalline particles which cause tiny faults in bulk glass, and measurements of crystalline coatings for texture, crystallite size and crystallinity.

Materials Science

Diffraction is used in the materials science discipline to examine crystal size, quantify microstrain, determine the dislocation density, measure crystallographic preferred orientation, quantify the volume fraction of phases within a specimen and also to determine the crystal structure of unknown samples such as corrosion products and new materials.