Energy ordering of grain boundaries in Cr₂O₃: Insights from theory

The grain boundaries (GBs) of corundum Cr₂O₃ are known to play an important role in the diffusion of ions within the oxide, which is an important phenomenon for the corrosion of stainless steels. The extent of the growth of oxide layers in stainless steel depends upon which interfaces are preferred within Cr₂O₃. Therefore, we have constructed four different grain boundary planes (rhombohedral, basal, prismatic and pyramidal) and their various associated interface symmetries known in literature for corundum Al₂O₃. Their structural, electronic, and energetic properties are investigated theoretically with periodic boundary conditions using the DFT + U approach. We find that the prismatic screw GB with a Cr-O plane interface is the energetically preferred GB with the rhombohedral GB with screw symmetry and Cr vacancy termination being the second energetically preferred GB. The increase of the number of in-plane Cr atoms at the interface of prismatic GB enhances the stability, which is also evident in the electronic density of states.