Structural and Electronic Properties of Frenkel and Schottky Defects at the MgO{100} Surface: Spin Polarization, Mid-Band Gap States, and Charge Trapping at Vacancy Sites

Recent experimental and simulation studies on the hydration of MgO suggest that physically and chemically induced surface defects strongly promote the reaction. The results of density functional theory calculations on the stability and the structural and electronic properties of Frenkel and Schottky defects at the MgO{100} surface performed in light of the surface chemistry of MgO are presented here. Comparison of calculated formation energies shows that Frenkel and Schottky defects are more likely to be formed at the surface than in the bulk. Frenkel adatoms were found to induce a strong local restructuring of surface atoms. The lowest energy configurations include spin-polarized and dumbbell-type reconstructions for Mg and O Frenkel adatoms. O Frenkel vacancies were observed to trap significant amounts of electronic charge. Analysis of the electronic density of states reveals that surface Frenkel defects introduce many electronic defect states in the wide band gap of perfect MgO. These findings are a strong indication that the defective MgO surface is not chemically inert and will more easily bind or dissociate molecules and ions.