  Interaction between hydrogen (H) and metals is central to many materials
problems of scientific and technological importance. Chief among them is the
development of H storage and H-resistant materials. H segregation or trapping
at lattice defects, including vacancies, dislocations, grain boundaries, etc,
plays a crucial role in determining the properties of these materials. Here,
through first-principles simulations, we propose a unified mechanism involving
charge transfer induced strain destabilization to understand H segregation
behavior at vacancies. We discover that H prefers to occupy interstitials with
high pre-existing charge densities and the availability of such interstitials
sets the limit on H trapping capacity at a vacancy. Once the maximum H capacity
is reached, the dominant charge donors switch from the nearest-neighbor (NN) to
the next-nearest-neighbor (NNN) metal atoms. Accompanying with this long-range
charge transfer, a significant reorganization energy would occur, leading to
instability of the H-vacancy complex. The physical picture unveiled here
appears universal across the BCC series and is believed to be relevant to other
metals/defects as well. The insight gained from this study is expected to have
important implications for the design of H storage and H-resistant materials.
