Structure of Synthetic K-rich Birnessites Obtained by HighTemperature Decomposition of KMnO4. 2 Phase and Structural Heterogeneities

Synthetic K-rich birnessites (KBi) were prepared from the thermal decomposition of a fine-grained KMnO4 powder heated in air atmosphere at temperatures ranging from 2001000°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°C, 800°C and 1000°C (referred to as KBi7, KBi8h and KBi10h) have been studied in details by chemical and thermal analysis and by simulation of the experimental powder XRD patterns. Two-layer orthogonal (2O), and hexagonal (2H) as well as 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 Mn 3+ cations and to the unique azimuthal orientation of Mn3+ octahedra which are elongated because of Jahn-Teller distortion. In the 2H and 3R polytypes, the layers have a hexagonal symmetry as they contain only Mn 4+- and vacant octahedra. As a result, their interlayers have a heterogeneous cation composition, because of the migration of Mn 3+ from the layers to the interlayers. In addition to the periodic KBi polytypes, KBi7 and KBi8h contain MLSs in which layer pairs of the 2H polytype are interstratified at random with those of the 3R or of the 2O polytype. Interstratification of incommensurate 2O and 2H structural fragments leads to peculiar diffraction effects and represents a new type of structural disorder in birnessites. The increase of temperature from 700°C to 1000°C is associated with the replacement of 3R/2H, 2H, and 2O/2H mixed-layered structures by the more stable 2O polytype. KBi10h consists of a mixture of a minor 2H phase with three 2O varieties having slightly different layer unit-cell parameters. This phase heterogeneity results from the partial disorder in the orientation of Mn 3+ octahedra. The average structural formulae, K + 0.265Mn 3+ 0.145(Mn 4+ 0.825