= Occupational diffuse scattering
Static fluctuations in which <chemical element> occupies a site produce scattering beyond coherent <Bragg diffraction> from the mean <crystal structure>. For independent binary occupations with <probabilities> $p$ and $1-p$, the <variance> of the site's <atomic scattering factor> is $p(1-p)|f_A-f_B|^2$. Writing each site amplitude as its mean plus a zero-mean fluctuation shows that the mean squared total amplitude is the coherent contribution plus a term proportional to the number of sites and this <variance>. Consequently a forbidden reflection of the average <crystal structure> can still sit on a diffuse background. Correlated occupations add nontrivial <scattering vector> dependence.
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