Structure of the synthetic K-rich phyllomanganate birnessite obtained by high-temperature decomposition of KMnO 4: Substructures of K-rich birnessite from 1000 °C experiment

Abstract

The structure of a synthetic potassium-rich birnessite prepared from the thermal decomposition of KMnO 4 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 Mn layer 3 + cations within these layers being the main source of their deficit of charge, which is compensated for by interlayer K + cations. Mn 3+ 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, Mn 3+ octahedra are segregated in Mn 3+-rich rows parallel to the b axis that alternate with two Mn 4+ rows according to the sequence ⋯–Mn 3+–Mn 4+–Mn 4+–Mn 3+–⋯ along the a direction, thus leading to a A = 3 a super-periodicity. At 350 °C, the structure partially collapses due to the departure of interlayer H 2O molecules and undergoes a reversible 2 O to 2 H phase transition. This transition results from the relaxation of the cooperative Jahn–Teller effect, that is from the random orientation of elongated Mn 3+ octahedra.