Phase Determination of Bragg Reflections by Wavelength-Modulated Diffraction

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X-ray diffraction method has long been used as a method of structure determination, but there exists a serious problem called "phase problem". That is, although it is possible to know the amplitude of Bragg reflections from measured intensity, it is not possible to get information on the phase of Bragg reflections. Taking advantage of high brightness and wavelength tunability of synchrotron radiation, this research was performed to develop a new method of phase determination by wavelength modulated diffraction. System most suitable to this purpose was constructed at the beamline BL-1 in the SR Center at Ritsumeikan University and software to handle measured data was developed. In the system, the double-crystal monochromator is repeatedly rotated so that the wavelength of incident radiation is continuously changed over the range in the vicinity of the absorption edge of an atom contained in a crystal, while the sample crystal is rotated synchronously to reflect the radiation of varying wavelength. Diffraction pattern is recorded on an imaging plate, in which Bragg reflections are seen as elongated spots with an intensity profile showing positive or negative gradients at the absorption edge. By solving simultaneous linear equations with measured intensity gradients as constant terms, the phase can be derived. Feasibility experiments using a ferrocene derivative crystal with the iron atoms as anomalous scatters was carried out with successful results.