Rietveld refinement and experimental determination of optical and electrical properties of K⁺ stabilized α-MnO₂ nanostructures

In the midst of strenuous efforts in fabricating materials for supercapacitors, lithium ion batteries, molecular sieves and catalysts, manganese dioxide plays a vital role due to their unique tunnel framework. Alpha phased manganese dioxide (α-MnO₂) nanoparticles are synthesized using a low temperature coprecipitation method. The structural and morphological characterization carried out by XRD, SEM and HRTEM techniques, confirms the formation of α-MnO₂ nanoparticles. The EDS results confirmed the non stoichiometry and the presence of K⁺ ions within the crystal lattice. The non stoichiometry is due to the large number of oxygen vacancies within the crystal. Rietveld refinement of XRD pattern was carried out to study in detail the role of K⁺ ions in maintaining the stability of the 2x2 tunnel framework of α-MnO₂. The UV-Vis absorption and photoluminescence spectrum were studied for analyzing the energy band structure of MnO₂. A direct band gap of 1.66 eV is obtained from the famous Taucs plot. The photoluminescence exhibits strong emission line in the visible region at 743 nm and defect related emission peaks at 757 and 751 nm. The dielectric studies were carried out using dielectric spectroscopy. The temperature dependence of orientation polarization can be understood from the study of variation of dielectric constant and losses with temperature. Moreover, the phase purity of the sample and the Mn-O vibrational frequencies were analyzed using FTIR spectrum. (paper)