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

Highly crystalline α-MnO 2 nanotubes with outer diameters between 25 and 40 nm are formed by a hydrothermal decomposition of KMnO 4 precursor in an acidic environment. For nanotube formation, it is essential to include an addition of Fe 3+ 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 (∼370 nm). The intercalation of K + and Fe 3+ ions into the α-MnO 2 structure determines the local Mn 3+ /Mn 4+ 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 Mn 3+ ( t 2g 3 e g 1 ) to a non-active Mn 4+ ( t 2g 3 e g 0 ) site is responsible for the activated type of conductivity in mixed Mn 3+ /Mn 4+ 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 Mn 3+ /Mn 4+ and nearly pure Mn 4+ regions. At low temperatures, an antiferromagnetic ordering occurs at T N = 13.6 K, which is substantially lower than in bulk α-MnO 2 phases.