The high energy of hydrogen vibrations in solids is the origin of their strong impact on thermodynamic properties. The peculiar structure of complex hydrides amplifies this impact. We shed light on the vibrational properties of three allotropes of Ca(BH₄)₂ using density-functional theory calculations, infrared spectroscopy, and inelastic neutron scattering. We show that the vibrational properties of Ca(BH₄)₂ depend on the specific phase and are hitherto the origin of their differences in stability.

Metal borohydrides are potential materials for solid state hydrogen due to their high gravimetric and volumetric hydrogen densities. Among them, Ca(BH₄)₂ is particularly interesting because of the predicted suitable thermodynamic properties. In this work, we investigate a new synthesis route using high pressure reactive ball milling. Starting from CaH₂ and CaB₆ with a TiCl₃ or TiF₃ as additive, a reaction yield of 19% is obtained after 24 h milling at room temperature and 140 bar H₂. The presence of Ca(BH₄)₂ is confirmed by the presence of the stretching mode of the [BH₄]^- group in the infrared spectra of the as-milled samples. Using in-situ XRD, we observe the recrystallisation of a poorly crystallised Ca(BH₄)₂ phase present after milling. The reversible decomposition/formation of Ca(BH₄)₂ is obtained with higher yield (57%) using higher temperature and TiF₃ as additive but not with TiCl₃ despite its similar electronic structure. The differences observed using different additives and the influence of the anion are discussed.