Structure of Synthetic K-Rich Birnessites Obtained by High-Temperature Decomposition of KMnO4. 2. Phase and Structural Heterogeneities

Abstract

Synthetic K-rich birnessites (KBi) were prepared from the thermal decomposition of a fine-grained KMnO4 powder heated in an air atmosphere at temperatures ranging from 200 to 1000 °C. The qualitative analysis of powder X-ray diffraction (XRD) patterns reveals a complex range of structural transformations from one metastable phase to the other, often through intermediate mixed-layer structures (MLSs). Phase and structural heterogeneities of KBi samples synthesized at 700, 800, and 1000 °C (referred to as KBi7, KBi8h, and KBi10h) have been studied in detail by chemical and thermal analysis and by simulation of the experimental powder XRD patterns. Two-layer orthogonal (2O), two-layer hexagonal (2H), and three-layer rhombohedral (3R) polytypes were identified in these samples. The 2O structure consists of vacancy-free layers and their orthogonal symmetry is linked to the high content of layer Mn3+ cations and to the unique azimuthal orientation of Mn3+ octahedra which are elongated because of Jahn−Teller dis...