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

A procedure is presented for modelling the axial divergence aberration prof ile, taking into account all second-order and end effects of the diffractom eter system. It is shown that relatively simple calculations, which could b e introduced into Rietveld refinement programs, allow the instrument line p rofile to be modelled accurately at low and high scattering angles. This mo del 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].