Manganese oxides synthesized via microwave-assisted hydrothermal method: phase evolution and structure refinement

Manganese oxides were synthesized during 40 min at 140 deg C via Microwave-Assisted Hydrothermal (MAH) method and treated at different temperatures in order to evaluate the phase evolution using structure refinement (Rietveld method). The samples obtained were heat treated at temperatures defined by means of thermal analysis (160 deg C, 480 deg C, 715 deg C, 870 deg C, 920 deg C and 1150 deg C) and analyzed by X-Ray Diffractometry (XRD), X-Ray Fluorescence (XRF), Fourier Transform Infrared (FTIR) spectroscopy, Raman scattering, UV-Vis absorption and Scanning Electron Microscopy (SEM). Structural characterizations allowed to identify five distinct phases: α-MnO₂, Mn₃O₄, Mn₅O₈, Na₂Mn₅O₁₀ and Na₄Mn₉O₁₈ with weight percentages dependent on the heat treatment. The hausmannite structure (average crystallite size ranging from 28.9 nm to 99.1 nm) is present in all samples and go through various oxidation and reduction processes from 160 deg C to 1150 deg C without any major variation in the lattice parameters. Chemical characterizations identifies the presence of Na⁺ions in all samples, either as substitution defects or as components of specific crystalline structures (Na₂Mn₅O₁₀ and Na₄Mn₉O₁₈), showing that the synthesized manganese oxides works as Na⁺ intercalation compounds, important materials for energy storage devices optimization. The results presented enables a better interpretation of the thermal and structural characteristics of manganese oxides synthesized via MAH. (author)