Abstract
A computer software package, RIETAN, has been developed for the Rietveld refinement of angle-dispersive X-ray and neutron data, and TOF neutron data. The program is written in FORTRAN 77 and of modular construction to facilitate changes. RIETAN incorporates data bases of information on space groups, atomic scattering factors, scattering lengths, and anomalous dispersion corrections. It can handle all space groups without additional programming and allows refinement of the diffraction data of samples containing two or more phases. A new profile shape function, which is the sum of a Gauss function and a Lorentz function with unequal peak heights and widths at one-half peak height, has been included for the analysis of angle-dispersive diffraction data. Three different algorithms for nonlinear least squares have been implemented: Gauss-Newton, Marquardt, and conjugate direction methods. One can easily change the method of least squares one after another during refinement. These three methods are combined with an exterior penalty function method so that objective functions can be minimized under equal and unequal constraints which may be linear and/or nonlinear functions with respect to parameters. The program can not only plot the results of Rietveld analysis but also simulate powder diffraction patterns by using user-supplied parameters.
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