Tritium Diffusion Behavior In Solid Breeder Materials Of Li₂TiO₃

Tritium,as an isotope of hydrogen,is an important luminescent marker and one of the raw materials for nuclear fusion.And the solid breeder materials are the only raw materials for producing tritium.Li₂TiO₃ has good chemical stability and tritium release properties,which is considered as one of the most promising candidate materials for solid breeder materials.However,the micro-mechanism of tritium diffusion behavior in Li₂TiO₃ is still unclear so far.In this paper,the tritium diffusion behavior and optical properties of Li₂TiO₃have been systematically studied by first principles method from the atomic scale.The crystal structure of Li₂TiO₃ is optimized and analyzed.By self consistent computation and non self consistent computation.The band gap of Li₂TiO₃ crystal is obtained as 3.101 eV,which indicates that it has the potential to be used as a wide band gap semiconductor luminescence material.Different defect models are further built up and optimized.It is found that Ti vacancy is the most difficult defect to form in the Li₂TiO₃ crystal,while O vacancy is the second,and Li vacancy is the easiest defect to form.In addition,the 1/3-Li（001）surface is found to be the most stable surface of Li₂TiO₃ crystal.Based upon the optimized structures,construct the vacancy defect of Li₂TiO₃ crystal and the tritium adsorption in the Li vacancy,O vacancy and Ti vacancy of Li₂TiO₃ crystal and tritium trapping capability of each vacancy are studied,respectively.The results show that the Ti vacancy has the strongest trapping ability for tritium atoms,which can accommodate 4 tritium atoms.Only one tritium atom can be captured by the O and Li vacancies.Moreover,the tritium trapping capability of three vacancies under hydrostatic pressure is also studied.The results present that the adsorption energy of tritium in the vacancy decreases with the increasing hydrostatic pressure,and the tritium trapping ability of the vacancy is weakened.On the basis of the obtained stable adsorption sites,the tritium diffusion process on 1/3-Li（001）surface and diffusion process from inside of the crystal to this surface are studied.Tritium atoms are found to diffuse preferentially along the[100]direction on the surface and the pathways across the surface were identified.The corresponding energy barrier is about 0.5 eV,which is the lowest energy barrier for tritium diffusing on the surface.The energy barrier of tritium atom from the inside of the crystal to the surface is about 1.56 eV,which is much higher than that of tritium atom diffusion on the surface.This is mainly due to the fact that the interactions between tritium atom and neighboring oxygen atoms are weakened during the diffusion process from inside to the surface according to the analysis of local density of states and the charge distribution.Based on the above results,it is speculated that the tritium release behavior of Li₂TiO₃ can be further improved by defect adjustment and surface modification.The research model in this paper can also provide inspiration for the study of tritium release behavior of other solid breeder materials.