Vibrational Spectroscopic Study of Subtle Phase Transitions in Alkali Borohydrides: Comparison with First-Principles Calculations

Neutron vibrational spectra have been measured for the alkali borohydrides KBH4, RbBH4, and CsBH4. The BH4– torsional band for each compound changes noticeably across the corresponding low-temperature phase transition previously identified using thermodynamic (NaBH4, KBH4, RbBH4, and CsBH4) and crystallographic (NaBH4 and KBH4) techniques. Previous neutron diffraction measurements show that the transitions for both NaBH4 and KBH4 are order–disorder transitions involving the relative orientations of the BH4– anions. However, diffraction measurements for both RbBH4 and CsBH4 fail to unequivocally identify long-range-ordered phases below the transitions. The present measurements of BH4– torsional as well as translational optic bands across the transitions, corroborated by first-principles phonon calculations, suggest that the subtle RbBH4 and CsBH4 transitions are indeed analogous to those observed for NaBH4 and KBH4 but of shorter range.