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 °C) a finely ground KMnO 4 powder in acidified water was determined. At room temperature the structure of KBi corresponds to a 3 R - 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 √3 = 4.923 Å, b = 2.845 Å, γ = 90°, and c = 21.492 Å. The layer charge deficit originates exclusively from the presence of vacant layer sites as only Mn 4+ cations are present in the octahedral layers, and the following structural formula can be proposed: (M □ 0.074 )O 2 ·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 °C, except for the dehydration of KBi interlayers. Heating the KBi sample to 350 °C leads to a dramatic modification of its crystal chemistry resulting from the partial reduction of layer Mn 4+ cations associated with the departure of O layer atoms. Despite unfavorable steric strains induced by the Jahn−Teller distortion of Mn 3+ -octahedra, reduced Mn 3+ cations remain in the octahedral layers, leading to vacancy-free layers with the following structural formula: (M M □ 0.024 )O 2 ·0.42H 2 O. At 350 °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 Mn 3+ −O bond with respect to the a axis. After cooling, the structure of KBi becomes monoclinic (space group C 2/ m ) with a = 5.130 Å, b = 2.850 Å, c = 7.131 Å, and β = 101.0°. In this case steric strains are minimized by the segregation of Mn 3+ -octahedra in Mn 3+ -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.