Nature of Decahydro-closo-decaborate Anion Reorientations in an Ordered Alkali-Metal Salt: Rb2B10H10

The ordered monoclinic phase of the alkali-metal decahydro-closo-decaborate salt Rb2B10H10 was found to be stable from about 250 K all the way up to an order–disorder phase transition temperature of ≈762 K. The broad temperature range for this phase allowed for a detailed quasielastic neutron scattering (QENS) and nuclear magnetic resonance (NMR) study of the protypical B10H102– anion reorientational dynamics. The QENS and NMR combined results are consistent with an anion reorientational mechanism comprised of two types of rotational jumps expected from the anion geometry and lattice structure, namely, more rapid 90° jumps around the anion C4 symmetry axis (e.g., with correlation frequencies of ≈2.6 × 1010 s–1 at 530 K) combined with order of magnitude slower orthogonal 180° reorientational flips (e.g., ≈3.1 × 109 s–1 at 530 K) resulting in an exchange of the apical H (and apical B) positions. Each latter flip requires a concomitant 45° twist around the C4 symmetry axis to preserve the ordered Rb2B10H10 m...