Single Crystal Growth of Birnessite- and Hollandite-Type Manganese Oxides by a Flux Method

By using a KNO 3 flux, single crystals of birnessite- and hollandite- type potassium manganese oxides can be obtained. The formation reactions and products were investigated by XRD and Rietveld refinement, SEM observation, TG-DTA, and chemical analysis. KNO 3 takes on two roles in the formation of potassium manganese oxides, performing as a reactant supplying K + ions, and also as a solvent. Rod-shaped hollandite-type and hexagonal prismatic birnessite-type manganese oxide single crystals were obtained at low (500 and 600 °C) and high (700 °C) temperatures, respectively. The hollandite crystals have a tetrahedral symmetry with space group I 4/ m . The K + -form birnessite crystals have a hexagonal symmetry, and the presence of small unindexed peaks in the XRD pattern suggests the formation of superstructure. K + ions were extracted from the interlayer of the birnessite by the treatment in a (NH 4 ) 2 S 2 O 8 aqueous solution, and the resulting K + -extracted birnessite has a hexagonal symmetry with space group P 6 3 / mmc .