Q1(h). Powder X-ray diffraction for identifying solids

Answer in brief

Every crystalline phase gives its own set of d spacings and line intensities, a fingerprint that identifies the compound and its crystal form directly in the solid, non-destructively, by matching with the Powder Diffraction File.

Key points

• Every crystalline substance has its own set of interplanar spacings d and relative intensities; Bragg's law nλ = 2d sin θ converts 2θ to d.
• It identifies the compound and its crystalline form, not just the elements: e.g. calcite from aragonite.
• A mixture gives the superposed patterns of its components; each phase is identified separately.
• A few milligrams of powder suffice; no single crystal or dissolution needed; non-destructive and fast.
• Reference patterns are filed in the Powder Diffraction File (ICDD); matching by Hanawalt method or search–match.