It was noted that T ions and Li vacancies were created by neutron irradiation, and formed bulk defect complexes whose nature was experimentally unclear. Here,  ab initio  total-energy pseudopotential methods were used to study the structures and relative energies of T as a substitutional defect, and of separate T interstitials and Li vacancies. For all of the stable defect geometries, the formation of an OT- complex with an O-T bond-length of about 0.1nm was found to be energetically favored. In the case of the substitutional defect, this bond was found to point towards the vacant Li site, but the direction was relatively free for the interstitial case. The binding energy of T to a Li vacancy was found to be equal to 1.3eV. Structural relaxation effects were included, and were found to affect significantly the relative energies of various defect geometries. The effects of zero-point fluctuations were estimated and were found to be insignificant. The most probable migration path of interstitial T was identified to be a jump between nearest-neighbor O ions; with an activation energy of 0.45eV (in agreement with experiment). The results suggested the thermally assisted diffusion of T interstitials and Li vacancies through the anion and cation sub-lattices, respectively, with preferential trapping of the 2 defects into substitutional complexes.

Ab initio Study of Tritium Defects in Lithium Oxide. R.Shah, A.De Vita, M.C.Payne: Journal of Physics - Condensed Matter, 1995, 7[35], 6981-92