Darstellung und Reaktionen von Psilomelan (Romanéchit) Ba2Mn15O30∙4H2O

Abstract

Psilomelane Ba₂Mn₁₅O₃₀∙4H₂O was synthesized under hydrothermal conditions from Ba-7Å-phyllomanganate(IV) in BaCl₂ solution. The topotactic reaction transforms the initial layer lattice into needles with tunnels twice the size of hollandite type tunnels. These are filled with unexchangeable Ba²⁺ ions and water molecules which cannot be reversibly removed. Intergrowth with hollandite type microdomains is frequent and can be directly observed in the electron microscope. Thermal treatment of synthetic psilomelane leads via increasing intergrowth to hollandite BaMn₈O₁₆. Refluxing in acid produces after a month γ-MnO₂ (and, in the end, β-MnO₂). Synthetic psilomelane has no exchange properties. The M2 position in the lattice is occupied by Mn³⁺ ions which explain the elongated coordination octahedron as Jahn-Teller effect.