IMPROVED TREATMENT OF SEVERELY OR EXACTLY OVERLAPPING BRAGG-REFLECTIONS FOR THE APPLICATION OF DIRECT-METHODS TO POWDER DATA

A new method has been developed for the improved intensity assignment of severely or exactly overlapping Bragg reflections in a powder diffr action pattern. This fast iterative Patterson squaring (FIPS) method a ddresses, in particular, the systematic lack of small and large normal ized intensities (Absolute value of E values) in severely overlapping powder data, which causes (a) the intensity statistics to be strongly acentric (even when the structure itself is centrosymmetric) and (b) t he underestimation of strong structure-invariant relationships. Direct methods for structure determination are more likely to succeed with F IPS-improved data than with conventionally used equipartitioned data ( ratio of Absolute value of E values for overlapping reflections set to 1.0). In the case of the molecular sieve AFR (in which 65% of the ref lections severely overlap), the ab initio structure solution was only possible after a redistribution of the intensities by the FIPS method.