AB-INITIO STUDY OF TRITIUM DEFECTS IN LITHIUM-OXIDE

Lithium oxide has been suggested as a suitable breeder blanket materia l for fusion reactors. Tritium ions and lithium vacancies are created by neutron irradiation, forming bulk defect complexes whose exact char acter is experimentally unclear. We have used nb initio total energy p seudopotentiaI methods to study the structure and relative energies of tritium as a substitutional defect, and of the separate tritium inter stitial and lithium vacancy. For all stable defect geometries, the for mation of an OT- complex with an O-T bond length of about 1 Angstrom i s found to be energetically favoured. In the case of the substitutiona l defect this bond is found to point towards the vacant Li site, but t he direction is fairly free for the interstitial case. The binding ene rgy of tritium to a lithium vacancy is found to be 1.3 eV. Structural relaxation effects are included throughout, and are found to significa ntly affect the relative energies of different defect geometries. The effects of zero-point fluctuations are estimated and found not to be v ery significant. The most probable migration path of interstitial trit ium is identified as a jump between nearest-neighbour oxygen ions, wit h an activation energy of 0.45 eV, in agreement with experimental evid ence. The results suggest a picture of thermally assisted diffusion of tritium interstitials and lithium vacancies along the anion and catio n sublattices respectively, with the preferential trapping of the two defects into substitutional complexes.