Remarks on the geometric parameters in describing diffraction optics of asymmetric Bragg crystals

This communication gives the asymmetric factor b and deviation parameter η describing the diffraction geometry in a more general way including very asymmetric scattering arrangements. Darwin's parameter Y defined on the dispersion surface and the associated deviation parameter η_Y given by the Takagi–Taupin equations are proposed to characterize the angular deviation from the exact resonance of Bragg diffraction. With respect to η_Y of a high accuracy, the validity of conventional forms of the deviation parameter is further discussed.