Mn1.5Ga1.5O4 Spinel Decomposition Upon Heating in the Air: Effect of the Thermal Treatment Mode

The air stability of mixed oxide Mn1.5Ga1.5O4 with the spinel structure is studied by in situ X-ray diffraction (XRD), thermogravimetric analysis, and transmission electron microscopy. The Mn1.5Ga1.5O4 behavior is shown to depend on the thermal treatment mode, and correspondingly, kinetics of the process. During continuous heating to 650 °C initial oxide decomposes into two cubic spinels: Mn1.5Ga1.5O4 and (Mn,[·])1.5(Ga,[·])1.5O4. They differ in the oxygen concentration. The amount of attached oxygen increases during stepwise heating, which increases the number of cationic vacancies in the (Mn,[·])1.5(Ga,[·])1.5O4 phase and generates additional superstructural XRD peaks. The difference in transformations is due to oxygen attachment kinetics, and consequently, the formation of cationic vacancies. Thus, upon rapid heating the vacancies are randomly arranged in the mixed oxide bulk, whereas upon longer heating their agglomeration and ordering are observed. Hydrogen reduction treatment results in the inverse process: oxygen evolution and recrystallization of the initial spinel.