On the akaganéite crystal structure, phase transformations and possible role in post-excavational corrosion of iron artifacts

The combination of intense X-ray synchrotron radiation and an imaging-plate area detector makes it possible to extract real-time structural information using Rietveld refinement of individual time slices. Powder diffraction data from capillary samples were collected using a flat imaging plate mounted perpendicular to, or at an oblique angle to, the incoming beam. Owing to the geometry of the experiments, several factors must be taken into account when Rietveld refinement of powder diffraction data obtained using a flat imaging plate is performed: (i) nonequal step sizes are obtained when the diffraction profiles are extracted; (ii) the refined zero-point correction is different from that for conventional diffraction geometry; (iii) the sample-detector distance is not constant, requiring a modified geometric term in the Lorentz factor; (iv) the geometric contribution to the instrumental resolution function must be considered. The geometric factors have been derived and are discussed. The Rietveld program XRS -82 and the profile extraction program ALLHKL have been modified to allow flat imaging-plate powder diffraction data to be refined. The Rietveld refinement of powder diffraction data of α -quartz collected using a 30 s exposure is presented. LaB 6 powder diffraction data were collected using various settings. The profile variation with angle and the resolution function are described. Data were collected up to sin θ / λ = 1.29 Å −1 ( d values down to 0.39 Å) using high energy and with the imaging plate inclined at an angle to the incoming beam.