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.