Applying standard diffraction techniques to crystal powders or polycrystals is tantamount to collapsing the 3D reciprocal space, as obtained via single-crystal diffraction, onto a 1D axis. The resulting partial-to-total overlap of symmetry-independent reflections renders the structure determination process more difficult, if not impossible.
Powder diffraction data can be plotted as diffracted intensity (I) versus reciprocal lattice spacing (1/d). Reflection positions and intensities of known crystal phases, mostly from X-ray diffraction data, are stored, as d-I data pairs, in the Powder Diffraction File (PDF) database. The list of d-I data pairs is highly characteristic of a crystal phase and, thus, suitable for the identification, also called ‘fingerprinting’, of crystal phases.
Search-match algorithms compare selected test reflections of an unknown crystal phase with entries in the database. Intensity-driven algorithms utilize the three most intense lines (so-called ‘Hanawalt search’), while d-spacing-driven algorithms are based on the eight to ten largest d-spacings (so-called ‘Fink search’).
X-ray powder diffraction fingerprinting has become the standard tool for the identification of single or multiple crystal phases and is widely used in such fields as metallurgy, mineralogy, forensic science, archeology, condensed matter physics, and the biological and pharmaceutical sciences.