Modelling of line profile asymmetry caused by axial divergence in powder diffraction

A procedure is presented for modelling the axial divergence aberration profile, taking into account all second-order and end effects of the diffractometer system. It is shown that relatively simple calculations, which could be introduced into Rietveld refinement programs, allow the instrument line profile to be modelled accurately at low and high scattering angles. This model compares well with ray-tracing calculations and can give better results than the usually used model proposed by Van Laar & Yelon [J. Appl. Cryst. (1984), 17, 47-54] and Finger, Cox & Jephcoat [J. Appl. Cryst. (1994), 27, 892-900].