Synthesis of 3D Hierarchical Self-Assembled Microstructures Formed from α-MnO2 Nanotubes and Their Conducting and Magnetic Properties

Highly crystalline alpha-MnO2 nanotubes with outer diameters between 25 and 40 nm are formed by a hydrothermal decomposition of KMnO4 precursor in all acidic environment. For nanotube formation, it is essential to include all addition of Fe3+ ions in the reaction mixture. SEM and TEM Studies reveal that nanotubes are self-assembled into three-dimensional hollow microstructures where the shell thickness corresponds to the average nanotube length (similar to 370 nm). The intercalation of K+ and Fe3+ ions into the alpha-MnO2 Structure determines the local Mn3+/Mn4+ ratio, and with this, influences their electric conductivity and magnetic properties. At high, temperatures, the small polaron hopping of the e(g) electron from a Jahn-Teller-active Mn3+ (t(2g)(3)e(g)(1)) to a non-active Mn4+ (t(2g)(3)e(g)(0)) site is responsible for the activated type of conductivity in mixed Mn3+/Mn4+ regions with the activation energy E-a/k(B) = 2200 K. Magnetic susceptibility and EPR measurements show two distinct behaviors, Suggesting nanometric phase segregation into mixed Mn3+/Mn4+ and nearly pure Mn4+ regions. At low temperatures, all antiferromagnetic ordering occurs at T-N = 13.6 K. which is Substantially lower than in bulk alpha-MnO2 phases.