Molecular-dynamics simulation study of threshold displacements and defect formation in zircon

The propensity of a primary-knock-on-atom (PKA) to produce defects in zircon is investigated by molecular-dynamics simulations. The dynamic behavior of highly directed collision sequences in each of the sublattices is examined for a range of energies up to 200 eV. The energy range is limited by the system size, which in turn is limited by the long-range interactions of the potential model. For the heavier ions, both homogeneous and heterogeneous collision sequences lead to the dissipation of large amounts of energy and to defect formation. There is a large range of energy before the first displacement event occurs, which is much larger for the cations than for the anion, where the energy is absorbed into the lattice without damage formation. Above the minimum threshold displacement energy there is another range of energy up to 200 eV for which formation damage is relatively constant. The limitations in the system size, and therefore on the PKA energies, does not allow for a thorough search of a second distinct displacement event in the same direction. In some cases, no damage occurs for energies up to 200 eV. The oxygen anions are seen to have a stronger response to the motion of the silicon cations as compared to the motion of the zirconium cation due to the stronger binding energy of Si with O. Consequently, silicon atoms maintain their tetrahedral coordination, whereas zirconium atoms can reduce their coordination number without a great loss of stability.