Vacancy Formation Energies in Plastic Crystals Using Positron Annihilation Techniques

Abstract

Abstract Ortho-positronium annihilation has been used to assess vacancy formation energies and vacancy concentrations in several plastic crystalline solids. In general, the ortho-positronium lifetimes showed a gradual increase with temperature at low temperatures consistent with normal lattice expansion, coupled with sudden larger increases at the brittle-plastic phase transformation. In most solids, at about O.7Tm, a larger sigmoid increase occurred indicative of the trapping of ortho-positronium in thermally generated defects in the lattice. The lifetimes in the defect state are consistent with the conclusion that the defects are of a size equivalent to vacant lattice sites. On this basis and using the defect trapping model devised for metals, activation energies for vacancy formation were evaluated for adamantane, bicyclooctane, succinonitrile and dl-camphene. The data are consistent with a vacancy concentration of 10−7 to 10−3 vacancy fraction over the range 0. 7Tm-Tm (melting temperature)