Predicted Formation of H3(+) in Solid Halogen Polyhydrides at High Pressures.

The structures of compressed halogen polyhydrides HnX (X = F, Cl and n > 1) and their evolution under pressure are studied using ab initio calculation based on density functional theory. HnF (n > 1) are metastable up to 300 GPa, whereas for HnCl (n > 1), four new stoichiometries (H2Cl, H3Cl, H5Cl, and H7Cl) are predicted to be stable at high pressures. Interestingly, triangular H3(+) species are unexpectedly found in stoichiometries H2F with H3HF2, H3F with H3F, H5F with H3HF2[H2]3, and H5Cl with H3Cl[H2] above 100 GPa. Importantly, formation processes of H3(+) species are clearly seen on the basis of comparing bond lengths, bond overlap populations, electron localization functions, and Bader charges as a functions of pressure. Further analysis reveals that the formation of H3(+) species is attributed to the pressure-induced charge transfer from hydrogen atoms to halogen atoms.