Single crystal growth of birnessite- and hollandite-type manganese oxides by a flux method

By using a KNO3 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. KNO3 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 degreesC) and high (700 degreesC) temperatures, respectively. The hollandite crystals have a tetrahedral symmetry with space group I4/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 (NH4)(2)S2O8 aqueous solution, and the resulting K+-extracted birnessite has a hexagonal symmetry with space group P6(3)/mmc.