= Accidental diffraction extinction from equal atomic scattering factors
A vanishing <crystal basis structure factor> can depend on approximate equality of <atomic scattering factors>, rather than on an exact <space group> condition. For two inequivalent site classes, an amplitude $F=C(f_A-f_B)$ vanishes when $f_A=f_B$ but becomes weakly observable if bonding <electron densities> or thermal displacements differ. Chemical identity alone does not imply identical local <atomic scattering factors>. The <diffraction intensity> is quadratic in the small difference.
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