Interplay between the Reorientational Dynamics of the B(3)H(8) (-) Anion and the Structure in KB(3)H(8).

The structure and reorientational dynamics of KB(3)H(8) 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 [B(3)H(8)](-) anion undergoes different reorientational motions in the three polymorphs α, α', and β. In α-KB(3)H(8), the [B(3)H(8)](-) 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 [B(3)H(8)](-) anion upon transitioning to α'-KB(3)H(8). After transitioning to β-KB(3)H(8), the [B(3)H(8)](-) anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of β-KB(3)H(8) deviates from the global cubic NaCl-type structure. The results also indicate that the high reorientational mobility of the [B(3)H(8)](-) 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.