From Online Dictionary of Crystallography
Loi de Friedel (Fr). Friedelsche Gesetz (Ge). Ley de Friedel (Sp). Legge di Friedel (It)
Friedel's law, or rule, states that the intensities of the h, k, l and reflections are equal. This is only true if there is no absorption. It is in that case not possible to tell by diffraction whether an inversion center is present or not. The apparent symmetry of the crystal is then one of the eleven Laue classes.
The reason for Friedel's rule is that the diffracted intensity is proportional to the the square of the modulus of the structure factor, |Fh|2, according to the geometrical, or kinematical theory of diffraction. It depends similarly on the modulus of the structure factor according to the dynamical theory of diffraction. The structure factor is given by:
where fj is the atomic scattering factor of atom j, h the reflection vector and the position vector of atom j. There comes:
if the atomic scattering factor, fj, is real. The intensities of the h, k, l and reflections are then equal. If the crystal is absorbing, however, due to anomalous dispersion, the atomic scattering factor is complex and
Friedel's law therefore does not hold for absorbing crystals. The reflections h, k, l and are called a Friedel pair. They are used in the resolution of the phase problem for the solution of crystal structures.
Friedel's law was stated by G. Friedel (1865-1933) in 1913 (Friedel G., 1913, Sur les symétries cristallines que peut révéler la diffraction des rayons X., C.R. Acad. Sci. Paris, 157, 1533-1536.
Section 3.1 of International Tables of crystallography, Volume A