Chemically Assisted Precompression of Hydrogen Molecules in Alkaline-Earth Tetrahydrides

Through a series of high pressure diamond anvil experiments, we report the synthesis of alkaline earth (Ca, Sr, Ba) tetrahydrides, and investigate their properties through Raman spectroscopy, X-ray diffraction, and density functional theory calculations. The tetrahydrides incorporate both atomic and quasi-molecular hydrogen, and we find that the frequency of the intramolecular stretching mode of the $H_2^{\delta -}$ units downshifts from Ca to Sr and to Ba upon compression. The experimental results indicate that the larger the host cation, the longer the $H_2^{\delta -}$ bond. Analysis of the electron localization function (ELF) demonstrates that the lengthening of the H-H bond is caused by the charge transfer from the metal to $H_2^{\delta -}$ and by the steric effect of the metal host on the H-H bond. This effect is most prominent for BaH₄, where the precompression of $H_2^{\delta -}$ units at 50 GPa results in bond lengths comparable to that of pure H₂ above 275 GPa.