= Powder X-ray diffraction
<Powder X-ray diffraction> measures <X-ray diffraction> from many crystal grains with different orientations. For each allowed set of lattice spacings, suitable grains satisfy <Bragg's law> and produce a cone of diffracted directions, seen as a ring on an area detector or a peak in an angular scan. Several peak positions and systematic absences can identify a crystal structure even when a sample no longer retains one single-crystal orientation. Preferred orientation, strain and phase coexistence can complicate interpretation.
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