Friedel pairs are Bragg reflections related by inversion through the origin.
Friedel's Law states that members of a Friedel pair have equal amplitude and opposite phase.
Friedel's Law is broken whenever there is anomalous scattering.
If all atoms scatter equally, then the amplitudes remain equal
but the phase relationship no longer holds. This is because the
term is always positive.
If some atoms scatter anomalously and some don't, then both the
amplitude and phase relationships are broken.
X-ray Anomalous Scattering
Bijvoet Pairs