Indexing gets its name from the assignment of Miller index labels to each of the peaks in a pattern. ![]() XRPD indexing is the process of determining the size, shape and symmetry of the crystallographic unit cell for a crystalline component responsible for a set of peaks in an XRPD pattern. XRPD indexing is one method that can be used to extract information and aid the interpretation of XRPD patterns. Extracting information from an XRPD pattern beyond visual interpretation adds significant value and greatly enhances understanding of crystal forms. Despite the wealth of information available, routine interpretation of XRPD patterns is often limited to a qualitative visual comparison that, at best, underutilises the available information and, at worst, leads to incorrect conclusions. Information encoded in the XRPD pattern includes whether a material is a single phase or a mixture of phases the size, shape and symmetry of the unit cell the position of the molecules in the unit cell and the crystallite strain, among other information. The XRPD pattern of a crystal form at a given thermodynamic state point serves as a fingerprint for the form under the given conditions. For these reasons, XRPD is used routinely for the characterisation of crystalline solids. X-ray powder diffraction (XRPD) is an ensemble analysis that is generally representative of a powder material, and utilises powder samples that are often easier to produce than single crystals. ![]() Furthermore, a single crystal is not always representative of the polycrystalline material from which it was obtained. The main drawback to single crystal diffraction is the need to grow a sufficiently large and defect-free crystal for analysis, which is not practical for all crystal forms. A single crystal structure provides full structural characterisation of the form at the atomic scale. Crystal-structure determination using X-ray diffraction from single crystals is a well-developed technique (1).
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