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

Wavelength-modulated diffraction was developed by Iwasaki, Yurugi & Yoshimu ra [Acta Cryst. (1999), A55, 864-870] as a method for phase determination, in which the intensity of Bragg reflections is recorded using radiation who se wavelength is changing continually over a range in the vicinity of the a bsorption edge of an atom in the crystal. Using a ferrocene derivative crys tal (chemical formula C36H32O7Fe, space group P2(1)/a) with the Fe atoms ch osen as anomalous scatterers, measurements were made of the intensity gradi ent dI/d lambda of the reflections with an imaging plate as a detector on a synchrotron radiation source at Ritsumeikan University. In the case of a c entrosymmetric crystal, the phase of the structure factor could be derived by measuring only the sign of dI/d lambda at one wavelength in the range. O f 104 reflections measured, the correct phase was assigned to 101 reflectio ns. A discussion is given on the errors involved and on the limits of appli cation of the method.