From Online Dictionary of Crystallography
Paire de Friedel (Fr). Coppia di Friedel (It)
The couple of reflections h, k, l and is called a Friedel pair, or Bijvoet pair. Their intensities are equal if there is no absorption, but differ otherwise. Friedel's law then does not hold. Generally speaking, when absorption is present, equivalent reflections generated by the symmetry elements in the crystal have intensities different from those of equivalent reflections generated by the introduction of an additional inversion centre in normal scattering. Friedel, or Bijvoet pairs are used in the resolution of the phase problem for the solution of crystal structures.
The use of Friedel's pairs for helping in phase determinations was suggested by Bijvoet J. M., Peerdeman A. F. & van Bommel A. J. (1951), Determination of the absolute configuration of optically active compounds by means of X-rays. Nature (London), 168, 271–272.
Section 2.4.3 of International Tables of Crystallography, Volume B
Section 4.2.6 of International Tables of Crystallography, Volume C