)
)
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A direct method was tested in solving the incommensurate modulated structure of the Bi-2212 superconductor using simulating powder diffraction data.
Phase errors of direct-method phasing were calculated against the final structure model from single-crystal diffraction data. The statistics is shown in the following table, where the reflections were sorted in descending order of F(obs) and cumulated into groups. Within each cumulative group (a single row), N-ref is the number of reflections while N-err is the number of reflections having their signs wrongly derived by direct methods.
Direct-method phased Fourier maps are shown on the slices in comparison with that
from single-crystal data.
J.R. Chen, Y.X. Gu, C.D. Zheng & H.F. Fan, Chinese Physics
12, 1261-1265 (2003). "Ab-initio determination of the incommensurate modulated structure
of Bi-2212 from x-ray powder di raction data - a simulation"
)
A direct method was tested in solving a noncentrosymmetric structure with pseudo-translational symmetry using simulating powder diffraction data. (Results submitted for publication)
J.R. Chen, Y.X. Gu & H.F. Fan, Chinese Physics
12, 310-314 (2003). "Solving a superstructure from two-wavelength x-ray
powder di raction data - a simulation")
A method was proposed for ab-initio solution of noncentrosymmetric structures from 2-wavelength synchrotron X-ray powder diffraction data.
The data resembles a set of single-isomorphous replacement (SIR) data in single-crystal protein-structure analysis and suffers from the problem of phase ambiguity. Direct methods are used to break the phase ambiguity yielding a set of starting phases. The complete structure is then obtained by Fourier recycling. The procedure was verified with simulating powder diffraction data.
Y.X. Gu, Y.D. Liu, Q. Hao & H.F. Fan, Acta Cryst.
A56, 592-595 (2000). "Solving crystal structures from two-wavelength X-ray
powder diffraction data - breaking the phase ambiguity in the noncentrosymmetric
case"