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

Abstract

Manganese oxides were synthesized during 40 min at 140 ºC 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 ºC, 480 ºC, 715 ºC, 870 ºC, 920 ºC and 1150 ºC) and analyzed by XRD, XRF, FTIR spectroscopy, Raman scattering and 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 ºC to 1150 ºC without any major variation in the lattice parameters. The results presented enables a better interpretation of the thermal and structural characteristics of manganese oxides synthesized via MAH.