Structure of Synthetic K-Rich Birnessites Obtained by High-Temperature 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 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 distortion. In the 2H and 3R polytypes, the layers have a hexagonal symmetry as they contain only Mn4+ and vacant octahedra. As a result, their interlayers have a heterogeneous cation composition because of the migration of Mn3+ 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 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 Mn3+ octahedra. The average structural formulas, K+0.265Mn3+0.145A (Mn4+0.8250.175)O2·0.68H2O for KBi7 and KBi8h and K+0.27(Mn4+0.77Mn3+0.210.02)O2·0.53H2O for KBi10h, are in agreement with the main crystal chemical features of the phases prevailing in these samples. When heated to 350 C, the 2O polytype presents a hexagonal layer symmetry with a b parameter (2.894 A), which is significantly increased as compared to that determined at room temperature (2.850 A). Both modifications arise from random orientation of elongated Mn3+ octahedra along directions forming n60 angles with the a axis. The main factors responsible for the phase and structural heterogeneity of the KBi samples are discussed.