A treatment of the coplanar three-beam Si[000,440,404] diffraction

The peak of the spectral line Ni Kα₂ was measured by the double-crystal spectrometer in the arrangement (Si 440 sym., +Si 440 sym.). It is possible to adjust the Si crystal so that the conditions for coplanar three-beam Si[000, 440, 404] diffraction are satisfied for the wavelength of the incident radiation λ ~ 0.166288 nm involved in the Ni Kα₂ peak. When the second Si crystal was adjusted to this three-beam diffraction position an anomalous change of the measured dependence was found. A very simple model of the experiment was suggested and the necessary calculation completing the paper of Graeff & Bonse [Z. Phys. B, (1977), 27, 19-32] was performed to explain qualitatively the anomalous change.