Stability of a hydrogen molecule in a vacancy of palladium hydrides

We report our ab-initio calculations of energy states of equilibrium H–H separation in a vacancy of palladium and palladium hydrides at a variety of H/Pd loading ratios. In a vacancy of pure palladium, the H2 molecule has a shallow local energy minimum only in the [001] direction at a separation of 0.96 Å and it dissociates into positions near interstitial sites due to its high energy state. Increasing the H/Pd ratio to the beta phase deepens the energy well of the H2 molecule and results in a shorter H–H separation. At a loading ratio around 1, the H2 molecule is mostly affected by surrounding hydrogen neighbors and the H–H separation reaches 0.77 Å. The H2 molecule is then fairly stable and its energy state is comparable to that of nearby interstitial sites. Our calculations suggest that the loading ratio of hydrogen in palladium has a significant effect on the stability of the H2 molecule in the vacancy.