Friedel's law

"Friedel's law" states that the intensities of the hkl reflection and the -h-k-l reflection are equal even for polar crystals. This law holds for X-ray diffraction to which kinematical diffraction can be applied; however does not hold for electron diffraction where the dynamical diffraction effect is strong. This fact implies that X-ray diffraction cannot distinguish polar and non-polar crystals unless anomalous dispersion is utilized, but electron diffraction can.

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