= 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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