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.

Specifically labeled NaBD₃H has been synthesized and characterized using X-ray diffraction, NMR, and vibrational spectroscopy. The isotopic purity of the compound, as estimated from NMR spectra, was found to be about 85% with the compound NaBD₂H₂ as the second product. IR spectra confirm the relatively strong intensity of the single B–H stretching mode predicted from DFT calculations. Anharmonic DFT calculations show that for the BD₃H^- ion Fermi resonances with the single B–H stretching mode are very limited, making this mode a promising structural probe for complex borohydrides which can be prepared by metathetical reactions.

IR and Raman data were obtained from α-, β-, and mixed (β,γ)-Ca(BH ₄) ₂ samples and from the deuterated β,γ phase mixture. The results obtained with α phase indicate that the DFT calculated values for the B−H stretching modes and the lattice vibrations are fairly close to the experimental values. The spectral behavior at temperatures around the transition to the β phase shows a continuous transition and suggests the presence of disorder caused by reorientational motions of the [BH ₄] ^− ion in the β phase. The data indicate that there are more deformation bands observed for the mixed (β,γ) samples than for the α phase which indicates structural variations between the β and the γ phases.