# Ab initio study of hydrogen and helium diffusion in Be2Ti

## TL;DR

This study uses ab initio methods to analyze how hydrogen and helium diffuse in the Be2Ti compound, which is relevant for nuclear reactor materials.

## Contribution

The paper provides new insights into hydrogen and helium diffusion mechanisms and energy barriers in the Be2Ti compound.

## Key findings

- Be2Ti has three stable hydrogen and one stable helium interstitial sites with lower solution energies than pure beryllium.
- Hydrogen diffusion in Be2Ti has lower energy barriers than in pure beryllium, suggesting faster diffusion.
- Helium diffusion is controlled by inter-hexagonal jumps with a 0.52 eV energy barrier.

## Abstract

Interstitial hydrogen and helium diffusion in the Be2Ti compound was investigated via ab initio methods. Under certain conditions, this phase can coexist within the desired Be12Ti compound, which is a candidate neutron multiplier material for breeder blankets in the DEMO reactor. The Be2Ti lattice contains three stable interstitial hydrogen sites and one stable interstitial helium site, all exhibiting lower solution energies than those found in pure beryllium. This indicates a higher solubility of both hydrogen and helium in Be2Ti. Diffusion barriers between adjacent hydrogen/helium interstitial sites are calculated using a dimer method. At low concentrations, interstitial hydrogen predominantly diffuses through the energetically favorable interstitial sites A, forming a connected network, with an inter-hexagonal barrier of 0.19 eV. At higher concentrations and elevated temperatures, the diffusion involves less energetically favorable interstitial sites B and C, with higher energy barriers of 0.39 and 0.44 eV, respectively. Interstitial helium diffusion is controlled solely by inter-hexagonal jumps with a barrier of 0.52 eV, while the intra-hexagonal barrier is negligible. The energy barriers between adjacent non-equivalent interstitial hydrogen sites A are at least two times lower than the rate-limiting energy barrier in pure beryllium (0.42 eV), suggesting a higher diffusion rate in Be2Ti.

We have investigated the migration barriers and diffusion pathways for the interstitial hydrogen and helium diffusion in the Be2Ti phase.