State of a pressurized helium bubble in iron

We study the state of a nanometric helium bubble in bcc-iron as a function of temperature and He content using atomistic calculations. It appears that up to moderate temperatures the Fe lattice can confine He to solid state, in good agreement with known solid-liquid transition diagram of pure He. However, He in the bubble forms an amorphous phase, while an fcc structure is expected at the same temperature and He density. In addition, the He bubble forms a polyhedron whose morphology depends on either the surface energy or the elastic-plastic properties of Fe at either low or high pressure, respectively. Indeed, at high He contents the bubble surface breaks down at the mechanical stability limit of the Fe crystal, leading to a pressure decrease in the bubble.