Structure of birnessite obtained from decomposition of permanganate under soft hydrothermal conditions. 1. Chemical and structural evolution as a function of temperature

The structure of a synthetic K-rich birnessite (KBi) prepared by hydrothermally reacting (4 days at 170 degrees C) a finely ground KMnO4 powder in acidified water was determined. At room temperature the structure of KBi corresponds to a 3R(-) polytype which can be described as AbC(b')(a')CaB(c')(a')BcA(b')(c')AbC... using the close-packing formalism. Assuming an orthogonal base-centered unit cell, KBi has a = b root 3 = 4.923 angstrom, b = 2.845 angstrom, gamma = 90 degrees, and c = 21.492 angstrom. The layer charge deficit originates exclusively from the presence of vacant layer sites as only Mn4+ cations are present in the octahedral layers, and the following structural formula can be proposed: K-0.296(+) (Mn-0.926(4+)square(0.074))O-2 center dot 0.40H(2)O. The layer charge deficit is compensated by the sole presence of interlayer K in prismatic cavities located above (or below) an empty tridentate cavity of the lower (or upper) layer and below (or above) a Mn octahedron of the upper (or lower) layer. This site is shifted from the center of the prismatic cavity toward its faces. This structure is essentially unaffected when heating this sample up to 250 degrees C, except for the dehydration of KBi interlayers. Heating the KBi sample to 350 degrees C leads to a dramatic modification of its crystal chemistry resulting from the partial reduction of layer Mn4+ cations associated with the departure of O-layer atoms. Despite unfavorable steric strains induced by the Jahn-Teller distortion of Mn3+ -octahedra, reduced Mn3+ cations remain in the octahedral layers, leading to vacancy-free layers with the following structural formula: K-0.312(+)(Mn0.7614+Mn0.2153+square(0.024))O-2 center dot 0.42H(2)O. At 350 degrees C the partial reduction of Mn-layer leads to a dramatic increase of the lateral dimensions of the layer arising from the random orientation of the long Mn3+-O bond with respect to the a axis. After cooling, the structure of KBi becomes monoclinic (space group C2/m) with a = 5.130 angstrom, b = 2.850 angstrom, c = 7.131 angstrom, and beta = 101.0 degrees. In this case steric strains are minimized by the segregation of Mn3+-octahedra in Mn3+-rich rows parallel to the b axis and by the systematic orientation of their long Mn-O bonds along the a axis. The origin of the above structural modifications and the nature of the stacking disorder are also determined.