A random procedure for the decomposition of a powder pattern in EXPO

The main obstacle to ab initio crystal structure solution by powder data is the lack of accuracy in the single Bragg integrated intensity estimates. T he two-stage method proves powerful for solving crystal structures from pow der data: in the first stage, the integrated intensities and, consequently, the structure-factor moduli, are extracted from the pattern for each refle ction; in the second stage, direct methods may be applied to estimate the p hases of the structure factors. The greater the accuracy of the integrated intensities, the more efficient the phasing procedure. A new procedure is d escribed, aiming at overcoming the tendency of the Le Bail algorithm [Le Ba il et al. (1988). Mater. Res. Bull. 23, 447-452] to equipartition the inten sities of strongly overlapping reflections. It proves to be efficient for l owering the phase error in the phasing process. The procedure has been impl emented in the EXPO program [Altomare et al. (1999). J. Appl. Cryst. 32, 33 9-340].