Past exam of the materials science course of the University of Cambridge / 2019 / Paper 1 / 3 / c / i / Solution

Summing phase-weighted atomic scattering factors over the four positions givesIf $h,k$ have different parity, the reflection vanishes. If they have equal parity, this becomes $2[f_{\mathrm{Au}}+(-1{)}^{h+l}{f}_{\mathrm{Cu}}]$. The difference reflections reveal chemical order; they disappear when all sites instead have the same average scattering factor.