PNNL

Abstract

Density functional theory simulations have been carried out to investigate the diffusion of interstitial tritium in Al12(TM)2.34 and Al2O bulk phases. In Al12(TM)2.34 the transition metal (TM) sites are occupied on average by 58.93 at.% Fe, 18.52 at.% Cr, and 22.54 at.% Ni. While the insertion of interstitial tritium in Al2O lead to a strong disordering of the structure, we only investigated interstitial tritium in Al12(TM)2.34 and its iron end-member (i.e., Al12Fe2.34). Nine diffusion pathways have been investigated along the a-, b-, and c-axis of Al12(TM)2.34. The direction of fastest diffusion for interstitial tritium is found to be along the b-axis, with a calculated diffusion coefficient of ˜10-10 m2.s-1 at 600 K, which is at least one order of magnitude faster than those previously calculated for interstitial tritium in other Al-rich phases such as Fe2Alx, Fe4Al13, and FeNiAl5 for which D_"T" =10-11 m2.s-1, ˜10-12 m2.s-1, and D_"T" ˜10-13 m2.s-1 respectively. The comparison of the energy landscape between Al12(TM)2.34 and Al12Fe2.34 for the fastest diffusion pathways in each axis direction, found that some pathways are not sensitive to transition metal mixing, while other are more affected, leading to higher energy barrier in Al12Fe2.34 in part due to a more energetically favorable formation of Fe—T bond compared to Ni—T. However, we found that both materials have similar diffusion coefficients as the fastest diffusion pathways occurs along pathways that are not very sensitive to transition metal mixing.