LIMITS ON PRECISION AND ACCURACY IN POWDER DIFFRACTION DATA ANALYSIS. W I F David, ISIS Science Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, U.K.

Powder diffraction is a powerful technique for the elucidation of increasingly complicated crystal structures. The present generation of high resolution X-ray and neutron powder diffractometers permits not only the refinement but also the ab-initio structure solution of complex inorganic materials, such as zeolites, and organic compounds, such as small drug structures with up to 30 independent non-hydrogen atoms. Indeed there is a belief that more intensity and more resolution will inevitably lead to the determination of more complex crystal structures from powder data. This talk will address the question of the limits of precision, accuracy and complexity available with current and next-generation diffractometers. The differences between precision and accuracy will be highlighted; situations where precision alone is necessary will be illustrated. Systematic errors will be shown to be the main source of loss of accuracy and emphasis will be stressed on care of sample preparation and the use of correct physical models to describe both samples and instrumental contributions to the diffraction profile. Neutron powder diffraction, with its intrinsically fewer systematic errors will be shown to offer the most accurate results. Optimised data collection methods for X-ray diffraction will be described that attempt to minimise systematic errors and produce precise results.