Structure of synthetic K-rich birnessite obtained by high-temperature decomposition of KMnO4.: I.: Two-layer polytype from 800°C experiment

The structure of a synthetic potassium birnessite (KBi) obtained as a finely dispersed powder by thermal decomposition of KMnO4 at 800 degreesC 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) Angstrom and c = 14.03(1) Angstrom and space group P6(3)/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 O-layer atoms with neighboring Mn-layer octahedra to form a triple-corner surface complex ((TC)-T-VI sites) and (2) by the presence of interlayer K in prismatic cavities located above or below empty tridentate cavities, sharing three edges with neighboring Mn-layer octahedra ((TE)-T-VI sites). As compared to the structure model proposed by Kim et al., this (TE)-T-VI 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.