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.
Mishra, A., Muramatsu, T., Liu, H., Geballe, Z., Somayazulu, M., Ahart, M., Baldini, M., Meng, Y., Zurek, E., & Hemley, R. (2018). New Calcium Hydrides with Mixed Atomic and Molecular Hydrogen. Journal of Physical Chemistry C, 122(34), 19370-19378. https://doi.org/10.1021/acs.jpcc.8b05030