Solid-state synthesis of ZnMn2O4 spinel: Sequence of phase transformations, thermal stability, localization and charge state of manganese ions in the intermediate and final reaction products

Abstract

This paper presents data on the solid-state synthesis of spinel ZnMn2O4, one of the leading promising materials for numerous practical applications. The evolution of the phase composition and charge state of manganese ions during the solid-state synthesis of ZnMn2O4 in air from ZnO and Mn2O3 has been studied. Using differential thermal, thermogravimetric, and X-ray diffraction analysis methods it has been established that, at a temperature of about 700 °C, an insignificant amount of ZnMn2O4 is present in the mixture with the initial oxides. During further heat treatment ZnMn2O4 interacts with Mn2O3 to form the solid solution based on ZnMn2O4. In parallel with this process, in the temperature range of 800–850 °C, ZnMn2O4 reacts with ZnO to form the compound ZnMn4+O3. It has been shown that heat treatment at 880–1180 °C leads to the formation of the final single-phase product – ZnMn2O4. Using diffuse reflectance spectroscopy, XPS, and FT-IR methods, the charge state and localization of Mn ions in the intermediate and final synthesis products have been determined. It has been shown that in the Zn1-xMn2+xMn3+2O4 solid solution Mn2+ ions are localized in the tetrahedral positions and Mn3+ ions in the octahedral positions of the structure. In the final single-phase product, ZnMn2O4, the tetrahedral positions are occupied only by zinc ions, and the octahedral positions are occupied only by Mn3+ ions. It has been established that ZnMn2O4 is stable up to 1300°С. Upon reaching this temperature, ZnMn2O4 decomposes into ZnO and Zn1-хMn2+хMn3+2O4.