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 hk 0 and 0 kl zones by means of two relationships among the Q (= 1/ d 2 ) values: 2 Q 020 + Q h 10 = Q h 30 and 3 Q 020 + Q h 20 = Q h 40 . The monoclinic angle is determined by trying certain low‐index h 0 l 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.