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 KMnO 4 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 KBi 7, KBi 8h, and KBi 10h ) have been studied in detail by chemical and thermal analysis and by simulation of the experimental powder XRD patterns. Two-layer orthogonal (2 O ), two-layer hexagonal (2 H ), and three-layer rhombohedral (3 R ) polytypes were identified in these samples. The 2 O 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 Mn 3+ octahedra which are elongated because of Jahn−Teller distortion. In the 2 H and 3 R 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, KBi 7 and KBi 8h contain MLSs in which layer pairs of the 2 H polytype are interstratified at random with those of the 3 R or of the 2 O polytype. Interstratification of incommensurate 2 O and 2 H 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 3 R /2 H, 2 H, and 2 O /2 H mixed-layered structures by the more stable 2 O polytype. KBi 10h consists of a mixture of a minor 2 H phase with three 2 O 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 formulas, K + 0.265 Mn 3+ 0.145 (Mn 4+ 0.825 □ 0.175 )O 2 ·0.68H 2 O for KBi 7 and KBi 8h and K + 0.27 (Mn 4+ 0.77 Mn 3+ 0.21 □ 0.02 )O 2 ·0.53H 2 O for KBi 10h, are in agreement with the main crystal chemical features of the phases prevailing in these samples. When heated to 350 °C, the 2 O polytype presents a hexagonal layer symmetry with a b parameter (2.894 Å), which is significantly increased as compared to that determined at room temperature (2.850 Å). Both modifications arise from random orientation of elongated Mn 3+ 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.