Diffusion and reactivity of ZnO–MnOx system

The solid state interaction between ZnO and MnOx in air was investigated at different temperatures by means of the diffusion couple technique. No diffusion is observed at temperatures below 973 K. Above this temperature, Mn(IV) is already reduced to Mn(III) and the subsequent formation of Mn2O3 impels the diffusion of manganese into the ZnO pellet. However, it never enters the wurtzite lattice, so no homogeneous Mn:ZnO solid solution is formed. Simultaneously, Zn greatly diffuses in the manganese pellet, and as a consequence, a new phase layer develops at MnOx/reaction zone interface. A mixture of cubic and tetragonal spinel-type phases initially comprises this layer. However at higher temperatures, the tetragonal ZnMn2O4 spinel is the unique phase present in the interface, and it forms a physical barrier for further diffusion of both zinc and manganese species in the respective pellets of the couple. Differences arising between ZnO, MnO2 and Mn2O3 crystal structures are behind these diffusion behaviors.