Ab initio study of intrinsic defects and diffusion behaviors in solid molecular hydrogens

The behaviors of intrinsic defects in solid molecular hydrogens (H 2 ) were investigated using ab initio calculations based on density functional theory. The results show that the formation energy of a vacancy is dependent on molecule orientation in disordered hexagonal close-packed (hcp) H 2 crystals, but independent of molecular orientation in face-centered cubic-Pa3 H 2 . Furthermore, H 2 molecules generally prefer to occupy the basal octahedral sites in hcp and octahedral sites in Pa3 structures. The formation energies of an interstitial H 2 depend sensitively on the volume of interstitial sites, and also on near spatial distributions of molecular axes of a H 2 in hcp H 2 crystals. The strong force field introduced by an interstitial H 2 might induce the rotation of molecular axes and reduce the formation energy of the interstitial H 2 . The migration barrier energy of a vacancy in Pa3 is larger than the average in hcp structures. However, the H 2 molecules prefer to jump though vacancies in the two structures of solid H 2 than the interstitial migrations.