Combined Pairs of Vacancies in Copper

An approximate theoretical calculation is given of the energy of dissociation of vacancy pairs in copper and of the activation energy required for the motion of associated pairs. If two atoms have been removed from neighboring lattice sites, the resulting double vacancy may be more stable than the configuration consisting of two single vacancies far apart. On the assumption that the lattice energy may be regarded as composed of contributions from pairs of atoms, the energy of dissociation of a double vacancy in copper has been estimated as about 0.6 ev. Furthermore, if the interatomic potential energy be represented by a Morse function, the activation energy for the motion of a double vacancy through any face-centered cubic metal crystal is about one-half that for a single vacancy. Double vacancies are, therefore, expected to be stable and highly mobile in such metals.