PHASE DETERMINATION AND EXTENSION USING X-RAY MULTIPLE DIFFRACTION AND MAXIMUM ENTROPY METHOD. Shih-Lin Chang*# and Chien-Mei Wang*, *Department of Physics, National Tsing Hua University, Hsinchu 30043, Taiwan, R.O.C., #Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan, R.O.C.

We develop a new scheme which combines the multiple diffraction technique with the maximum entropy method so as to extend the phases of structure factors and solve the current phase problem. In this approach, we use the known structure of the organic crystal C₂₅H₂₅NO₂ as the example. The organic crystal has four molecules(Z=4) in a unit cell and belongs to the space group P2₁2₁2, orthorhombic, with the cell dimensions 20.30 x 14.86 x 6.98ų. We experimentally determined 77 individual phases from 90 experimental triplet phases calculated from 215 measured psi-scan 3-beam and 4-beam diffraction profiles obtained with a rotating-anode x-ray source and synchrotron radiation, where the psi-scans were around the reciprocal lattice vectors of the (001), (002), and (003) reflections.The experimentally determined individual phase, used as the starting phases , are treated as the input to the process of estimating the values of probable structure factors, both magnitudes and phases, via the optimization of the entropy. Based on this electron density distribution, we refine the phases related to the experimentally determined individual phases with the maximum entropy method constrained by the triplet phases.With this approach,we have so far succeeded in phase extension up to more than a hundred reflections.