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

International audienceThe 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) Ä, and c = 14.03(1) Ä, and space group P63/mmc. In contrast to the structure model proposed by Kim et al., 1 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. This layer charge deficit is compensated for 1) by the presence of interlayer Mn 3+ above or below vacant layer octahedra sharing three O layer 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., 1 this VITE site is shifted from the center of the prismatic cavity towards its edges. A complementary powder XRD study confirmed the structure model of the main defect-free KBi phase and allowed to determine the nature of stacking disorder in a defective accessory KBi phase admixed to the defect-free KBi