Low- and high-temperature oxidation of Mn1.5Al1.5O4 in relation to decomposition mechanism and microstructure

Abstract

Spinel-like Mn_1.5Al_1.5O₄ is an unstable compound whose decomposition is induced by partial oxidation of Mn²⁺ ions in the temperature range of 300–800 °C in air. According to XRD, two spinel-like phases, Mn_0.4Al_2.4O₄ and Mn_2.8Al_0.2O₄, are formed during both cooling and heating of Mn_1.5Al_1.5O₄. In this paper, we have studied microstructural changes of Mn_1.5Al_1.5O₄ to understand its decomposition mechanism. During heating, low-temperature oxidation takes place and decomposition proceeds via a nucleation and growth mechanism. As a result slightly Al-doped β-Mn₃O₄ nanoparticles are formed on the surface of the parent spinel particles. On the contrary, cooling leads to high-temperature oxidation that results preferably in spinodal decomposition and formation of alternating Mn- and Al-rich lamellas of nanosized thickness. The present study provides a fundamental reference for the nanostructure design of a Mn–Al–O system and probably some other ones subjected to oxidative decomposition.