Influence of crystal structure on the CoII diffusion behavior in the Zn1-xCoxO system

The solid state interaction of the Zn1-xCoxO nominal system is investigated by means of diffusion couples and analysis of co-precipitated samples. The formation of a homogeneous Co:ZnO solid solution is found to be determined by the crystal structure from which CoII ions diffuse into the wurtzite lattice. No diffusion is observed whenever the CoO rock-salt structure is formed from the CoII precursor. On the contrary, the diffusion from the Co3O4 spinel phase is feasible but has a limited temperature range defined by the reduction at a high temperature of CoIII-CoII, since this process again leads to the formation of the rock-salt structure. However, when using a highly reactive and homogeneous co-precipitated starting powder, neither the spinel phase nor the rock-salt structure is formed, and a CoII:ZnO solid solution is obtained, which remains stable up to high temperatures. - Graphical abstract: Maximum diffusion distance for the ZnO-CoOx couple as a function of temperature. Dashed gray lines represent the temperature values at which the transformations between CoO and Co3O4 compounds take place