Pressure Effects on the Behavior of Helium in Niobium

By combining pseudopotential plane-wave method at 0 K with Car-Parrinello molecular dynamics at finite temperature, the behavior and aggregation of helium in niobium are simulated under different pressures. The result shows that the closed-shell helium atom plays a role of rigid-ball such that to destroy local metallic bonds of niobium and push away the near free electrons. Under ambient pressure and room temperature, locally high concentrated helium atoms immigrate towards the vacancy site, resulting in helium bubbles. The same helium density of He will dislocate Nb atom, forming an interstitialvacancy pair for the helium bubble formation. Under high pressures up to 40 GPa, no bubble is observed in perfect Nb host lattice due to the failure of interstitial-vacancy formation.