First-Principles Thermodynamics Study of Spinel MgAl2O4 Surface Stability

The surface stability of all possible terminations for three low-index (100, 110, 111) structures of spinel MgAl2O4 was studied using a first-principles-based thermodynamic approach. The surface Gibbs free energy results indicate that the 100_AlO2 termination is the most stable surface structure under ultrahigh vacuum at T = 1100 K regardless of an Al-poor or Al-rich condition. With increasing oxygen pressure, the 111_O2(Al) termination becomes the most stable surface in the Al-rich condition. The oxygen vacancy formation is thermodynamically favorable over the 100_AlO2, 111_O2(Al), and (111) structures with Mg/O connected terminations. On the basis of the surface Gibbs free energies for both perfect and defective surface terminations, 100_AlO2 and 111_O2(Al) are the most dominant surfaces in Al-rich conditions under atmospheric conditions. This is also consistent with our previously reported experimental observation.