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 (C1̄), 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 ∼2/3 Mn⁴⁺ and ∼1/3 Mn³⁺. The interlayer Na cations and H₂O molecules occupy a split site that shows evidence of considerable disorder.