Synthesis of NaxMnO2+δ by a reduction of aqueous sodium permanganate with sodium iodide

Reduction of sodium permanganate with sodium iodide in aqueous solutions has been investigated systematically. The products formed have been characterized by X-ray diffraction, wet-chemical analysis, and surface area and magnetic susceptibility measurements after firing at various temperatures. The results reveal that the sodium content x in the reduction products NaxMnO2+delta depends strongly on the reaction pH and mildly on the relative concentrations of the reactants. Na0.5MnO2+delta obtained at pH > 11 followed by firing at T > 500 degreesC adopts the P2 layer structure (hexagonal) with cation vacancies arising from a delta approximate to 0.3, Na0.7MnO2+delta crystallizing in a distorted P2 structure (orthorhombic) without cation vacancies (delta approximate to 0) could be obtained by annealing the hexagonal Na0.7MnO2+deltaf (delta approximate to 0.3) in N-2 atmosphere around 600 degreesC. While the orthorhombic Na0.7MnO2+delta (delta < 0.05) is stable during ion-exchange reactions with lithium salts at 25 T less than or equal to 180 degreesC, the hexagonal Na0.7MnO2+delta (delta approximate to 0.3) transforms to spinel-like phases due to the presence of cation vacancies. Na0.5MnO2+delta obtained at a controlled pH of 9.3 adopts a metastable layer structure on firing at 500 degreesC and a tunnel structure isostructural with Na4Mn4Ti5O18 on firing at T greater than or equal to 600 degreesC. The tunnel structure is stable to ion-exchange reactions without transforming to spinel-like phases. In addition, mashing the reduction products with various organic solvents before firing at higher temperatures is found to influence the reaction kinetics, composition, and crystal chemistry. (C) 2001 Academic Press.