Rietveld refinement of a triclinic structure for synthetic Na-birnessite using synchrotron powder diffraction data

Rietveld refinement using synchrotron powder X-ray diffraction data revealed that the crystal structure of synthetic Na-birnessite is triclinic (C (1) over bar), not monoclinic as was previously reported. The Mn-O octahedra have elongated axial bonds, consistent with Jahn-Teller distortion resulting from partial occupancy by Mn3+. Mean Mn-O distances indicate that Mn sites are similar to2/3 Mn4+ and similar to1/3 Mn3+. The interlayer Na cations and H2O molecules occupy a split site that shows evidence of considerable disorder. (C) 2002 International Centre for Diffraction Data.