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 °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) A and c = 14.03(1) A 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 wit...