Structure of the synthetic K-rich phyllomanganate birnessite obtained by high-temperature decomposition of KMnO4

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 3+ layer 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 = 3a super-periodicity. At 350°C, the structure partially collapses due to the departure of interlayer H 2 O 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 Mn 3+ octahedra.

Ba and Ni speciation in a nodule of binary Mn oxide phase composition from Lake Baikal

“…The d-spacings of the 2 0 À1 1 and 0 2 À3 1 reflections are in the ratio of ffiffi ffi 3 p , which indicates that the phyllomanganate layers have hexagonal symmetry with unit cell parameters a = b $ 2.83 Å . The low value of the layer-cell dimension is an indication that the layers do not contain detectable Mn 3+ Gaillot et al, 2007), in agreement with the oxidizing sample environment. As a comparison, synthetic hexagonal birnessite (HBi) equilibrated at pH 4 has 13% Mn 3+ in its layer and a and b parameters of 2.848 Å (Lanson et al, 2000).…”

Structure of nanocrystalline phyllomanganates produced by freshwater fungi

“…As an exception to this generality, orthogonal layer symmetry was observed in the Mn oxide produced by Bacillus sp. strain SG-1, as a result of the abundance and ordering of Mn 3+ cations Webb et al 2005;Gaillot et al 2007). The layer charge deficit resulting from vacant octahedral sites is balanced essentially by interlayer Mn 3+ when the medium is poor in trace metals; otherwise divalent metals, such as Pb, Co, Cu, Ni, and Zn, can compete positively against Mn 3+ (McKenzie 1989;Manceau et al , 2007aManceau et al , 2007bLanson et al 2002a;Tani et al 2004b;Tebo et al 2004;Peacock and Sherman 2007;Peacock 2009).…”

Half‐Metallicity in Single‐Layered Manganese Dioxide Nanosheets by Defect Engineering

“…[24,25] Furthermore, MnO 2 can be easily exfoliated into ultrathin nanosheets owing to the layered structure of the manganese oxide precursors, and defects can be readily introduced into the as-prepared nanosheets during the synthetic processes. [25][26][27][28] Herein, single-layered MnO 2 nanosheets with Mn vacancies were synthesized by exfoliation of their bulk counterparts in solution, as reported previously. [27,28] In detail, a defective layered manganese oxide precursor (K x MnO 2 ) was first prepared by decomposition of KMnO 4 at high temperature.…”

“…[25][26][27][28] Herein, single-layered MnO 2 nanosheets with Mn vacancies were synthesized by exfoliation of their bulk counterparts in solution, as reported previously. [27,28] In detail, a defective layered manganese oxide precursor (K x MnO 2 ) was first prepared by decomposition of KMnO 4 at high temperature. Then, the as-obtained K x MnO 2 was exfoliated into singlelayered nanosheets by proton exchange.…”

“…The concentration of Mn vacancies for the as-prepared nanosheets can be easily tuned by controlling the temperature at which K x MnO 2 is prepared in the first step, the lower the temperature, the more vacancies exist. [28] The morphology and structure of the as-obtained product were studied in detail. As can be seen from the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images (Figure 1 a, b), the as-obtained product shows sheet morphology with a size ranging from several hundred nanometers to a few micrometers.…”