The Vacancy Formation Energy in Crystalline Adamantane Determined by Positron Annihilation Techniques

The injection of positrons into crystalline adamantane results in the formation of positronium (Ps). At temperatures T> 400 K the mean lifetime of the long-lived ortho-positronium exhibits a sigmoidal increase with temperature characteristic of the trapping of this species in thermally created defects. The variation is well described by the trapping model usually applied to the trapping of positrons in defects in metals. The formation energy of the defects which trap ortho-Ps is estimated at Efv = 70 ± 10 kJ mol-1 (0.73 ± 0.1 eV). This value is in good agreement with estimates of the formation energy of lattice vacancies in adamantane derived from self-diffusion studies.