DATA COLLECTION, ANALYSIS AND ACCURACY IN SYNCHROTRON X-RAY POWDER DIFFRACTION. D. E. Cox, Physics Department, Brookhaven National Laboratory, Upton, NY 11973, USA

As more and more dedicated high-resolution powder diffraction beamlines become operational at synchrotron x-ray sources and the number of users continues to grow, it is increasingly important to optimize procedures for data collection and analysis, to minimize systematic errors such as inadequate powder randomization, preferred orientation and microabsorption, and to improve the overall level of accuracy. Prior to data collection, careful consideration must be given to specimen preparation, diffraction geometry, choice of wavelength and counting statistics. The peak profiles should be carefully examined for anisotropic sample broadening effects and lattice distortions; pattern fitting by the LeBail method [1] is strongly recommended at this stage, especially since the resulting profile parameters can then be fixed. During structure refinement, an attempt must be made to judge whether the introduction of additional structural parameters results in a significant improvement to the fit.

A summary will be given of some of the lessons learned at beamline X7A at the Brookhaven National Synchrotron Light Source during the past ten years. Some emphasis will be given to reference standards, data collection with capillary specimens, the use of short wavelengths (<= 0.7Å), absorption corrections, anisotropic peak-broadening, the accuracy of structural parameters and occupancy factors for light atoms, and significance tests.

[1] A. Le Bail, H. Duroy and J. L. Fourquet, Mater. Res. Bull. 23, 447 (1988).

This work was supported by the US Department of Energy, Division of Materials Sciences, under contract no. DE-AC02-76CH00016.