Phase determination by wavelength-modulated diffraction. I. Centrosymmetric case

Wavelength-modulated diffraction was developed by Iwasaki, Yurugi & Yoshimura [Acta Cryst. (1999), A55, 864–870] as a method for phase determination, in which the intensity of Bragg reflections is recorded using radiation whose wavelength is changing continually over a range in the vicinity of the absorption edge of an atom in the crystal. Using a ferrocene derivative crystal (chemical formula C₃₆H₃₂O₇Fe, space group P2₁/a) with the Fe atoms chosen as anomalous scatterers, measurements were made of the intensity gradient dI/dλ of the reflections with an imaging plate as a detector on a synchrotron radiation source at Ritsumeikan University. In the case of a centrosymmetric crystal, the phase of the structure factor could be derived by measuring only the sign of dI/dλ at one wavelength in the range. Of 104 reflections measured, the correct phase was assigned to 101 reflections. A discussion is given on the errors involved and on the limits of application of the method.