A method for analyzing powder patterns of phases of low symmetry

A procedure for determining cell parameters from Debye-Scherrer intensities is described which has been successful in analyses of triclinic phases resisting previous methods of attack. It minimizes the difference between observed and calculated powder pattern profiles, the principal elements of novelty being several new strategies for avoiding entrapment in false local minima while steering the refinement in the proper direction. A preliminary model of the distribution of atoms in the cell is required to enable the computation of intensities. So far the method has been applied only to molecular crystals for which trial molecular coordinates could be generated with the aid of an energy minimization algorithm. Besides its discrimination against false minima, the procedure offers the advantages that provisional assignments of reflections never need to be made, and the overlapping of reflections, even in severe cases, causes no particular trouble.