Structure of the synthetic K-rich phyllomanganate birnessite obtained by high-temperature decomposition of KMnO4 -: Substructures of K-rich birnessite from 1000°C experiment

The structure of a synthetic potassium-rich birnessite prepared from the thermal decomposition of KMnO4 at 1000 degrees C in air has been refined by Rietveld analysis of the powder X-ray diffraction (XRD) data, and the structure model shown to be consistent with extended X-ray absorption fine structure data. K-rich birnessite structure is a two-layer orthorhombic polytype (2O) with unit-cell parameters a = 5.1554(3) angstrom, b = 2.8460(1) angstrom, c = 14.088(1) angstrom, alpha = beta = gamma = 90 degrees, a/b = root 3.281, and was refined in the Ccmm space group. The structure is characterized by the regular alternation of octahedral layers rotated with respect to each other by 180 degrees. Octahedral layers are essentially devoid of vacant sites, the presence of 0.25 Mn-layer(3+) cations within these layers being the main source of their deficit of charge, which is compensated for by interlayer K+ cations. Mn3+ octahedra, which are distorted by the Jahn-Teller effect, are systematically elongated along the a axis (cooperative Jahn-Teller effect) to minimize steric strains, thus yielding an orthogonal layer symmetry. In addition, Mn3+ octahedra are segregated in Mn3+-rich rows parallel to the b axis that alternate with two Mn4+ rows according to the sequence ...-Mn3+-Mn4+-Mn4+-Mn3+-... along the a direction, thus leading to a A = 3a super-periodicity. At 350 degrees C the structure partially collapses due to the departure of interlayer H2O molecules and undergoes a reversible 2O to 2H phase transition. This transition results from the relaxation of the cooperative Jahn-Teller effect, that is from the random orientation of elongated Mn3+ octahedra. (C) 2006 Elsevier Inc. All rights reserved.