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 Xray diffraction. The results reveal the existence of a previously unknown polymorph in between the -and -polymorphs. Furthermore, it was found that the [B3H8] anion undergoes different reorientational motions in the three polymorphs , , and . In -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 C2 symmetry axis of the [B3H8] anion upon transitioning to -KB3H8. After transitioning to -KB3H8, the [B3H8] anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of -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.
