Interplay between the Reorientational Dynamics of the BH Anion and the Structure in KBH.

The structure and reorientational dynamics of KBH were studied by using quasielastic and inelastic neutron scattering, Raman spectroscopy, first-principles calculations, differential scanning calorimetry, and 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 [BH] anion undergoes different reorientational motions in the three polymorphs α, α', and β. In α-KBH, the [BH] anion performs 3-fold rotations in the plane created by the three boron atoms, which changes to a 2-fold rotation around the symmetry axis of the [BH] anion upon transitioning to α'-KBH. After transitioning to β-KBH, the [BH] anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of β-KBH deviates from the global cubic NaCl-type structure. The results also indicate that the high reorientational mobility of the [BH] 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.