New Calcium Hydrides with Mixed Atomic and Molecular Hydrogen

Two new polyhydrides of calcium have been synthesized at high pressures and high temperatures and characterized by Raman spectroscopy, infrared spectroscopy, and synchrotron X-ray diffraction. Above 20 GPa and 700 K, we synthesize a phase having a monoclinic (C2/m) structure with Ca₂H₅ composition, which is characterized by a distinctive vibration at 3789 cm^-1 at 25 GPa. The observed Raman spectrum is in close agreement with first-principles calculations of a Ca₂H₅ structure characterized by a lattice containing a central layer of H₂ molecules oriented along the (100) direction. At higher pressures (e.g., 116 GPa and 1600 K), we synthesize another phase, which has the composition of CaH₄ and a denser body-centered tetragonal structure. This weakly metallic phase also contains molecular-like H₂ units, and its spectroscopic as well as diffraction signatures match closely with those predicted from first-principles calculations. This phase is observed to persist on decompression to 60 GPa at room temperature. The elongation of the H-H bond in these hydrides is a result of the Ca-H₂ interaction, analogous to what occurs in molecular compounds, where H₂ binds side-on to a d-element, such as in Kubas complex.