Friedel's law

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Friedel's law, named after Georges Friedel, is a property of Fourier transforms of real functions.[1]

Given a real function , its Fourier transform

has the following properties.

where is the complex conjugate of .

Centrosymmetric points are called Friedel's pairs.

The squared amplitude () is centrosymmetric:

The phase of is antisymmetric:

  • .

Friedel's law is used in X-ray diffraction, crystallography and scattering from real potential within the Born approximation. Note that a twin operation (a.k.a. Opération de maclage) is equivalent to an inversion centre and the intensities from the individuals are equivalent under Friedel's law.[2][3][4]


  1. ^ Friedel G (1913). "Sur les symétries cristallines que peut révéler la diffraction des rayons Röntgen". Comptes Rendus. 157: 1533–1536.
  2. ^ Nespolo M, Giovanni Ferraris G (2004). "Applied geminography - symmetry analysis of twinned crystals and definition of twinning by reticular polyholohedry" (PDF). Acta Crystallogr A. 60 (1): 89–95. doi:10.1107/S0108767303025625.
  3. ^ Friedel G (1904). "Étude sur les groupements cristallins". Extract from Bullettin de la Société de l'Industrie Minérale, Quatrième série, Tomes III et IV. Saint-Étienne: Societè de l'Imprimerie Thèolier J. Thomas et C.
  4. ^ Friedel G. (1923). Bull. Soc. Fr. Minéral. 46:79-95.