Solving Oxide Structures by Precession Electron Diffraction

High resolution powder diffraction was used to characterize a sample of Ag(pyz)2S2O8.Measurements were taken at beamline X16c at the National Synchrotron Light Source at Brookhaven National Laboratory.The sample was found to contain multiple phases.Subsequent measurements, several months later, showed that a number of diffraction peaks had decreased dramatically in intensity.This is indicative that one phase had partially transformed, presumably into another, already present, phase.The disappearing peaks were indexed as c-centered monoclinic, with lattice parameters a = 15.969Å b = 7.133 Å c = 14.586Å β = 124.971°,and a candidate structure found.Due to the presence of multiple phases in the sample, however, ordinary Rietveld refinement of this structure was impossible.A robust refinement 1 procedure was implemented to refine the structure of the disappearing phase in the presence of other, unknown, phases.This was done using the software TOPAS Academic by continuous readjustment of the weights used in the figure of merit calculations, effectively changing χ 2 into its robust equivalent.The refined structure contains 4-coordinated silver, bonded in a square planar configuration to the nitrogen atoms of the pyrazine rings.The peroxodisulfate ions are located between the sheets of Ag(pyz)2.The robustly refined structure will be presented, and compared to previous knowledge of this material.