TWO-DIMENSIONAL DIFFRACTION DATA COLLECTION WITH A LARGE-VOLUME PRESS AT THE NSLS. Jiuhua Chen, Rui Li, John B. Parise and Donald J. Weidner, Center for High Pressure Research (CHiPR) and Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100

In situ structure determination and refinement is a growing trend in the high-pressure diffraction community. To obtain this crystallographic information, accurate diffraction-intensity data combining with existing d- space data is indispensable. Combination of a two-dimensional detector, an imaging plate, with a DAC has shown a great potential in collecting the accurate intensity data.

To take the advantages of a large-volume press, a quasi-hydrostatic pressure environment, large amount of diffracting sample, easy heating, and low temperature gradients, the imaging plate was applied to the DIA-type apparatus SAM85 at the X17B beamline of NSLS at Brookhaven National Laboratory. A high-pressure cell assembly with a disk-type heater was designed to minimize the number of materials in the x-ray path, and a subtraction technique was applied to eliminate the extrinsic peaks and background from the diffraction pattern. Typical exposure time is 5 min. Powder diffraction experiments were carried out on Ni-Mg olivine and NiA_l2O₄ spinel samples aiming to obtain the site-occupation information in these crystals. Structure refinements based on these observed data showed a obvious favor ordering of the cation distribution in these systems at high pressure. A kinetic phenominon of cation redistribution was observed in solid solution system of Ni-Mg olivine at 800 C and 4 GPa in 70 min.