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

Wavelength‐modulated diffraction was developed by Iwasaki, Yurugi & Yoshimura [ Acta Cryst. (1999), A 55 , 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 C 36 H 32 O 7 Fe, space group P 2 1 / a ) with the Fe atoms chosen as anomalous scatterers, measurements were made of the intensity gradient d I /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 d I /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.