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

International audienceThe structure of a synthetic K-rich birnessite (KBi) prepared by hydrothermally reacting (4 days at 170°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 using the close-packing formalism. Assuming an orthogonal base-centered unit cell, KBi has a = b√3 = 4.923 A, b = 2.845 A, γ = 90° and c = 21.492 A. The layer charge deficit originates exclusively from the presence of vacant layer sites as only Mn AbC...BcACaBAbC c' b' a' c' a' b' 4+ cations are present in the octahedral layers, and the following structural formula can be proposed