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 °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) Å, b = 2.8460(1) Å, c = 14.088(1) Å, α = β = γ = 90°, a/b = √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°. Octahedral layers are essentially devoid of vacant sites, the presence of 0.25 Mnlayer3+ 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 °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.