Ab initio study of tritium defects in lithium oxide

Abstract

Lithium oxide has been suggested as a suitable breeder blanket material for fusion reactors. Tritium ions and lithium vacancies are created by neutron irradiation, forming bulk defect complexes whose exact character is experimentally unclear. We have used ab initio total energy pseudopotential methods to study the structure and relative energies of tritium as a substitutional defect, and of the separate tritium interstitial and lithium vacancy. For all stable defect geometries, the formation of an OT- complex with an O-T bond length of about 1 AA is found to be energetically favoured. In the case of the substitutional defect this bond is found to point towards the vacant Li site, but the direction is fairly free for the interstitial case. The binding energy of tritium to a lithium vacancy is found to be 1.3 eV. Structural relaxation effects are included throughout, and are found to significantly affect the relative energies of different defect geometries. The effects of zero-point fluctuations are estimated and found not to be very significant. The most probable migration path of interstitial tritium is identified as a jump between nearest-neighbour oxygen ions, with an activation energy of 0.45 eV, in agreement with experimental evidence. The results suggest a picture of thermally assisted diffusion of tritium interstitials and lithium vacancies along the anion and cation sublattices respectively, with the preferential trapping of the two defects into substitutional complexes.