Interplay between the Reorientational Dynamics of the B₃H₈ ^- Anion and the Structure in KB₃H₈

The structure and reorientational dynamics of KB₃H₈ were studied by using quasielastic and inelastic neutron scattering, Raman spectroscopy, first-principles calculations, differential scanning calorimetry, and in situ synchrotron radiation powder X-ray diffraction. The results reveal the existence of a previously unknown polymorph in between the α'- and β-polymorphs. Furthermore, it was found that the [B₃H₈]^- anion undergoes different reorientational motions in the three polymorphs α, α', and β. In α-KB₃H₈, the [B₃H₈]^- anion performs 3-fold rotations in the plane created by the three boron atoms, which changes to a 2-fold rotation around the C ₂ symmetry axis of the [B₃H₈]^- anion upon transitioning to α'-KB₃H₈. After transitioning to β-KB₃H₈, the [B₃H₈]^- anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of β-KB₃H₈ deviates from the global cubic NaCl-type structure. The results also indicate that the high reorientational mobility of the [B₃H₈]^- anion facilitates the K⁺ cation conductivity, since the 2-orders-of-magnitude increase in the anion reorientational mobility observed between 297 and 311 K coincides with a large increase in K⁺ conductivity.