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 initio structure solution from powder data is demonstrated for the compound SrSO₄. This material is orthorhombic, in space group Pnma (a = 8.359, b = 5.351, c = 6.869 Å, Z = 4), with 11 variable atomic positional parameters, and gives a fairly complex powder pattern with many 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 uniquely resolved reflections, allowing a complete solution of the rest of the structure. Maximum entropy methods were found to be far superior to standard Fourier methods in the reconstruction of the electron-density distribution. The techniques of data collection, analysis and phase determination required for resonant scattering experiments of this type are described in detail.