One-dimensional α-MnO2:: Trapping chemistry of tunnel structures, structural stability, and magnetic transitions

Highly crystalline one-dirnensional (1D) alpha-MnO2 nanostructures were synthesized by a hydrothermal method. All samples were characterized by X-ray diffraction, transmission electron microscope, thermogravimetric and differential scanning calorimeter, and infrared spectroscopy. During the formation reactions, the tunnel structure of 1D alpha-MnO2 was simultaneously modified by NH4+ species and water molecules. The amount of NH4+ species that were trapped in the tunnels is almost independent on the reaction temperature, while the total water content increased with the reaction temperature. The average diameter of alpha-MnO2 nanorods increased from 9.2 to 16.5 nm when the reaction temperature increased from 140 to 220 degrees C. 1D alpha-MnO2 was destabilized by a subsequent high-temperature treatment in air. which is accompanied by a structural transformation to 1D Mn2O3 of a cubic structure. At low temperatures, all 1D alpha-MnO2 nanorods showed two magnetic transitions that were characterized by a decreased Neel temperature with rod diameter reduction. According to the effective magnetic moments experimentally measured, Mn ions presented in the nanorods were determined to be in a mixed valency of high spin state Mn4+/Mn3+. (C) 2007 Elsevier Inc. All rights reserved.