Elevated tritium diffusion barrier from varied segregation strengths in Li4TiO4 (1–10) grain boundary

Abstract

Whether the grain boundary (GB) accelerates or impedes the diffusion of hydrogen isotopes is reported with considerable inconsistency. Here the migration of tritium in (1–10) twin GB of Li₄TiO₄ breeder ceramic is investigated with density functional theory and kinetic Monte Carlo. The identified tritium sites along GB exhibit significantly varied segregation energies in the range of −1.57 ∼ −0.45 eV, which is attributed to the difference in the space volume around the site and the hydrogen bond. The minimum energy paths for tritium migration are obtained and the global barriers (1.12 eV and 0.88 eV along a and b, respectively) indicate a slower-than-bulk diffusion along GB. The rugged energy landscape along GB is analyzed by the kinetic and configurational components of local barriers. It's observed that the induced difference in configurational energies (i.e. the varied segregation strengths) by GB structure makes the major contribution to the raised global barrier. Diffusion coefficient for tritium migration along GB is estimated as $7.30\times 10^{-7}\text{exp}\left(-0.98\text{eV}/K_{B}T\right)m^2/s$, which is comparable to experimental data. The presented case offers insights for understanding the modified diffusion rates of segregated species by GB.