Akhtenskite-nsutite phases: Polymorphic transformation, thermal behavior and magnetic properties

Abstract

Two different MnO2 polymorphs, namely akhtenskite (ε-form) and nsutite (γ-form) were obtained from the comproportionation reaction between Mn2+ and MnO4−. The conditions that administer the transformation between both phases i.e. temperature, reflux time and concentration of reactants were explored. Other manganese oxide/hydroxide phases are obtained depending on variation of the pH of the medium or the solvent used in the reaction. Heat-treatment of both MnO2 polymorphs was done under isothermal and non-isothermal conditions, various pathways were adopted in both cases. The results of TGA/DTA analysis showed also that γ- MnO2 is more stable than ε- MnO2 and that ε-form contains much more excess mass over its surface (i.e. surface bound hydroxyl groups) than γ-form. Surface area measurements revealed that the reaction temperature has pronounced effect not only on the product phase but also on its surface properties. Very high surface area of 345 cm2/g was adopted by ε-MnO2 and decreases with increasing of reaction time (239 m2/g). SEM investigations showed that γ-MnO2 has a plate-like shape whilst ε-MnO2 exhibits a sponge-like morphology. Magnetic properties of both oxides were explored using SQUID magnetometer under zero field cooled and field cooled conditions. Both phases showed a ferromagnetic spin ordering with Curie temperature as 45 K for γ-form and 20 K for ε-form. The coercive force was found as 2500 and 2130 Oe, respectively. Remanent magnetization was 35 and 30 emu mol−1 for γ- and ε-MnO2, respectively.