Vibrational properties of MBH₄ and MBF₄ crystals (M = Li, Na, K): A combined DFT, infrared, and Raman study

In this work vibrational properties of alkaline-metal borohydrides and of the corresponding tetrafluorborates are studied by comparing DFT harmonic vibrational IR and Raman spectra of the crystals with the experimental ones, obtained by infrared attenuated total reflection and Raman techniques. The computed internal bending frequencies of the [BX₄]^- anions are found to be in good agreement with the experiment, and the computed stretching frequencies of tetrafluorborates are slightly underestimated. As expected, due to the neglecting of anharmonicity in the DFT spectra, the computed stretching frequencies of borohydrides are overestimated. The peak assignment of the experimental spectra is carried out in terms of factor group theory. For borohydrides, it is mostly in agreement with previously published data but for a peak observed at ca. 1400 cm^-1. The peak assignment for KBF₄ and LiBF₄ was carried out for the first time in terms of factor group theory. This work is the first step on the way to determining the vibrational properties of the MBH₄ + MBF₄ solid solutions for hydrogen storage materials with enhanced H ₂-release/uptake properties and solid-state electrolytes.