Stability Problems, Low-Energy-Recoil Events, and Vibrational Behavior of Point Defects in Metals

Abstract

In a copper crystallite containing many hundreds of atoms the static and dynamic behavior of point defects is simulated by means of an electronic computer. This follows the lines of an earlier work by Gibson et al. We have extended their investigation to collisions with subthreshold energies using a new computer program particularly suited for low-energy processes. The atomic interaction is approximated by a purely repulsive Born-Mayer potential together with surface forces applied to the boundary of the crystallite. The mechanical equilibrium of vacancy, interstitial, and many Frenkel-pair configurations has been determined. There are 74 lattice sites around a (split) interstitial which are unstable for a vacancy. The formation energy (2.79 eV) of a stable Frenkel pair turns out to be practically independent of the distance between vacancy and interstitial. The effect of subthreshold collisions on point defects has been studied. At low-energy transfers (\ensuremath{\lesssim} 0.3 eV) only vibrations can be excited. The (split) interstitial exhibits several localized modes: (i) an axial vibration, (ii) a twofold degenerate librational mode, and (iii) a twofold degenerate c. m. oscillation perpendicular to the axis. The energy storage in the axial mode is about 20% of the initial energy transfer. Jumps of point defects are induced by energy transfers of \ensuremath{\gtrsim} 0.3 eV for interstitials and \ensuremath{\gtrsim} 0.6 eV for vacancies. The directional dependence of these energy thresholds has been studied. The cross section for interstitial jumps due to MeV electrons is roughly ${10}^{+4}$ b. Effects of subthreshold energy transfers to close Frenkel pairs have also been investigated.