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Mechanisms of radiation damage of zircons deduced from computer simulation |
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| Authors: | V S Urusov A E Grechanovsky N N Eremin |
| Institution: | 1. Moscow State University, Moscow, 119992, Russia 2. Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017, Russia 3. Semenenko Institute of Geochemistry, Mineralogy, and Ore Formation, National Academy of Sciences, prosp. Palladina 34, Kiev-142, 03680, Ukraine |
| Abstract: | The radiation resistance of zircon (ZrSiO4) was comparatively tested with computer simulations of four different sets of parameters of interatomic potentials. The energies of Frenkel pairs (FP) for Zr, Si, and O atoms were calculated using the Mott-Littleton method in approximation of isolated defects. The difference between the FP energies calculated for four potentials and the FP energies obtained ab initio is the least in the one of the four calculated potentials that takes into account the ab initio data for Zircon 3. The formation of an atomic displacement cascade (ADC) after the passage of the initially knocked-out Th atom with energy of 20 keV was investigated using the molecular dynamics method. The number of FP in ADC reaches 5300 to 61 900, depending on the chosen potential. The number of FP at the end of the simulation varies from 480 to 4970. The distribution of internodal oxygen atoms in zircon has been studied. It is shown that within a time (t) interval of 0?C0.1 ps (ballistic stage), the internodal oxygen atoms knocked-out from the initial equilibrium site are predominant (O1 defects). Thereby the Si-O bond is ruptured. The mean displacement of those defects is 2?C3 ?. The probability of their survival is 34?C73% depending on the chosen potential. After t = 0.1 ps (onset of the thermal stage), many SiO4 tetrahedra in zircon are displaced with formation of a great amount of internodal oxygen atoms, because the Si-O bond is not ruptured. The mean displacement of those O2 defects is 1 ?; the probability of their survival is insignificant: 1.5?C3.0% depending on the chosen potential. The total amount of internodal oxygen atoms consists of O2 defects (19?C25%) and largely of O1 defects. A parameter characterizing that part of the energy of the initially knocked-out atom, which is consumed for FP, has been introduced. The physically acceptable values of this parameter (<1) are obtained only for Zircon 3 potential. The results show that a substantial discrepancy in relative amounts of Si and O atoms with displacements for 0.3?C0.5 ? from the equilibrium site exists for different potentials. In particular, the number of FP in zircon diminishes with decrease in mobility of atoms. It has been established that the lower the mobility of atoms, the lower the number of FP, and the most reasonable consistency with the preset conditions of the computer simulation is provided by Zircon 3 potential. |
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