Unexpected calcium polyhydride CaH₄: A possible route to dissociation of hydrogen molecules

Hydrogen-rich compounds provide an efficient route to pre-compressing hydrogen molecules and facilitating the creation of metallic hydrogen at much reduced pressure. Motivated by the long-sought theoretically proposed calcium hydrides, we have performed high-pressure experiments on the Ca-H system in a laser-heated diamond anvil cell. The unconventional compound CaH₄ with I4/mmm symmetry has been discovered to be stable above 25.5 GPa. Of particular significance is the crystal structure of CaH₄, which has an elongated H₂ molecular unit whose intramolecular bond strength changes with pressure. Below the dissociation pressure of pure hydrogen, the elongated H₂ unit is likely to dissociate into an atomic one. Our findings indicate that the presence of Ca atoms causes a very positive chemical pre-compression effect to potentially prompt the dissociation of the H₂ unit.