Systematic absence
= Systematic absence
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, $1+(-1)^{h+k+l}$ vanishes whenever $h+k+l$ 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.