Automated computer indexing of powder patterns: the monoclinic case

A procedure for the computer indexing of powder diffraction patterns of monoclinic crystals is described. The method is based on locating the orthogonal hk0 and 0kl zones by means of two relationships among the Q (= 1/d²) values: 2Q₀₂₀ + Q_h10 = Q_h30 and 3Q₀₂₀ + Q_h20 = Q_h40. The monoclinic angle is determined by trying certain low-index h0l and hkl values for the reflections not already indexed and by determining if the entire pattern can then be indexed. A computer program has been written in Fortran IV and extensively tested on a variety of known patterns of various degrees of experimental accuracy and inherent difficulty. At present, about 60% of the test patterns can be indexed in an automated fashion.