Structure of Synthetic K-rich Birnessite Obtained by High-Temperature Decomposition of KMnO₄. I. Two-Layer Polytype from 800 °C Experiment

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Abstract

The structure of a synthetic potassium birnessite (KBi) obtained as a finely dispersed powder by thermal decomposition of KMnO4 at 800 °C was for the first time studied by single-crystal X-ray diffraction (XRD). It is shown that KBi has a two-layer cell with a = 2.840(1) Å and c = 14.03(1) Å and space group P63/mmc. In contrast to the structure model proposed by Kim et al. (Chem. Mater. 1999, 11, 557−563), the refined model demonstrates the sole presence of Mn4+ in the octahedral layers, the presence of 0.12 vacant layer sites per octahedron being responsible for the layer charge deficit. In agreement with X-ray absorption spectroscopy result, this layer charge deficit is compensated (1) by the presence of interlayer Mn3+ above or below vacant layer octahedra sharing three Olayer atoms with neighboring Mnlayer octahedra to form a triple-corner surface complex (VITC sites) and (2) by the presence of interlayer K in prismatic cavities located above or below empty tridentate cavities, sharing three edges with neighboring Mnlayer octahedra (VITE sites). As compared to the structure model proposed by Kim et al., this VITE site is shifted from the center of the prismatic cavity toward its edges. A complementary powder XRD study confirmed the structure model of the main defect-free KBi phase and allowed for the determination of the nature of the stacking disorder in a defective accessory KBi phase admixed to the defect-free KBi.

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