THE APPLICATION OF RESONANT SCATTERING TECHNIQUES TO AB-INITIO STRUCTURE SOLUTION FROM POWDER DATA USING SRSO4 AS A TEST-CASE

The application of resonant (anomalous) scattering techniques to ab in itio structure solution from powder data is demonstrated for the compo und SrSO4. This material is orthorhombic, in space group Pnma (a = 8.3 59, b = 5.351, c = 6.869 Angstrom, Z = 4), with 11 variable atomic pos itional parameters, and gives a fairly complex powder pattern with man y overlapping peaks. Accurate high-resolution synchrotron X-ray powder diffraction data were obtained at three energies: 14, 225 and 5133 eV below the Sr K edge. From combinations of two of the three data-sets, partial Patterson densities were derived and used to phase the 71 uni quely resolved reflections, allowing a complete solution of the rest o f the structure. Maximum entropy methods were found to be far superior to standard Fourier methods in the reconstruction of the electron-den sity distribution. The techniques of data collection, analysis and pha se determination required for resonant scattering experiments of this type are described in detail.