W0452

X-Cell¹is a novel and robust indexing algorithm which employs an extinction-specific dichotomy procedure to perform an exhaustive search of parameter space to establish a complete list of all possible solutions. It can handle medium- to- high quality powder diffraction data obtained from X-ray, neutron, and electron radiation sources. X-Cell is the first indexing program to use explicit impurity tolerance levels to specify the maximum number of unindexed reflections. The dominant phase of mixtures can be indexed in the presence of up to 50% impurity peaks if high-quality synchrotron data are available. X-Cell searches from low to high peak numbers while explicitly considering systematic absences; thus providing for the correct unit cell to be found earlier. For a given range of allowed calculated peak numbers, a pattern of systematic absences, and an impurity tolerance level, an exhaustive search is performed. Zero-point shift is determined along with unit cell parameters. All indexing solutions are fully optimized and ranked according to a special figure of merit F_r, which favors solutions with low numbers of calculated peaks, adjustable parameters, and impurity peaks. Indexing of long and flat unit cells is facilitated by searching for rows (1D) or zones (2D) in reciprocal space first and then the search is refined using the lattice parameters of the dominant row or zone in the final unit cell (3D) search in combination with a modified Pawley refinement⁵. Initial validation¹ indicated that X-Cell has a higher success rate (92%) than DICVOL² (46%), TREOR³ (46%), and ITO⁴ (33%) for 24 cases. X-Cell copes very well with various difficulties typically encountered in powder indexing. Implementation of the successive dichotomy approach is significantly more efficient in X-Cell than in DIVOL¹.