A systematic absence is a class of diffraction reflections forced to vanish by symmetry or a repeated structural motif, rather than merely being weak by accident. It follows from exact cancellation in a crystal lattice structure factor or crystal basis structure factor within the stated scattering model.
For identical body-centering-related scatterers, vanishes whenever is odd. Changing the motif can lift the cancellation, as in chemical ordering and body-centering extinction. Additional effects such as unequal species amplitudes, disorder or multiple scattering can also produce intensity where a simplified model predicts zero. Reflection conditions therefore help infer structure, but an observed weak or missing spot needs a measurement and model qualification before it is treated as an exact symmetry statement.
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