Dynamical simulations of radiation damage and defect mobility in MgO

Collision cascades are investigated in MgO at energies ranging from 400 eV to 5 keV. Initial energy is imparted to the principle knock-on atom in the lattice and the cascade development is tracked using classical molecular dynamics. Temperature accelerated dynamics is performed on representative defects to follow the behavior to experimental time scales. Molecular statics is used to calculate basic properties of these defects, while density functional theory calculations are used to verify the potential. In the cascades performed at the lowest energy, the lattice either reforms perfectly or, if residual defects remain, these consist of isolated interstitials and vacancies and charge-neutral Mg-O divacancies and di-interstitials. As the energy is increased to 5 keV, isolated interstitials and di-interstitials remain the most common defects but more vacancy clustering can occur and interstitial defects consisting of up to seven atoms have been observed. Molecular statics calculations find that the binding energy per atom of the interstitial clusters increases from 3.5 to over 5 eV as the size increases from 2 to 16 atoms. Long-time-scale dynamics reveal that vacancies essentially never move at room temperature but that some interstitial clusters can diffuse quickly. Although tetrainterstitial interstitial clusters are essentially immobile, there is a long-lived metastable state of the hexainterstitial that diffuses one dimensionally on the nanosecond time scale at room temperature.