posted on 2003-12-02, 00:00authored byAnne-Claire Gaillot, David Flot, Victor A. Drits, Alain Manceau, Manfred Burghammer, Bruno Lanson
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.