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Nature of Decahydro-Closo-Decaborate Anion Reorientations in an Ordered Alkali-Metal Salt: Rb2B10H10



Mirjana NMN Dimitrievska, Vitalie Stavila, Alexei V. Soloninin, Roman V. Skoryunov, Olga A. Babanova, Hui Wu, Wei Zhou, Wan Si Tang, Antonio Faraone, Jacob Daniel Tarver, Benjamin Trump, Alexander V. Skripov, Terrence J. Udovic


The ordered monoclinic phase of the alkali-metal decahydro-closodecaborate salt Rb2B10H10 was found to be stable from about 250 K all the way up to an order-disorder phase transition temperature of approximately equal} 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 approximately equal}2.6x1010 s-1 at 530 K) combined with order-of-magnitude slower orthogonal 180° reorientational flips (e.g., approximately equal}3.1x109 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 monoclinic structural symmetry. This result is consistent with previous NMR data for ordered monoclinic Na2B10H10, which also pointed to two types of anion reorientational motions. The QENS-derived reorientational activation energies are 197(2) meV and 288(3) meV for the C4 four-fold jumps and apical exchanges, respectively, between 400 K and 680 K. Below the temperature range, NMR (and QENS) both indicate a shift to significantly larger reorientational barriers, e.g., 485(8) meV for the apical exchanges. Finally, subambient diffraction measurements identify a subtle change in the Rb2H10H10 structure from monoclinic to triclinic symmetry as the temperature is decreased from around 250 K to 210 K.
Journal of Physical Chemistry C


decahydro-closo-decaborate anion, neutron spin echo, nuclear magnetic resonance, NMR, quasielastic neutron scattering, QENS, Rb2B10H10, reorientational motions, rotations


Dimitrievska, M. , Stavila, V. , Soloninin, A. , Skoryunov, R. , Babanova, O. , Wu, H. , Zhou, W. , Tang, W. , Faraone, A. , Tarver, J. , Trump, B. , Skripov, A. and Udovic, T. (2018), Nature of Decahydro-Closo-Decaborate Anion Reorientations in an Ordered Alkali-Metal Salt: Rb<sub>2</sub>B<sub>10</sub>H<sub>10</sub>, Journal of Physical Chemistry C, [online], (Accessed June 25, 2024)


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Created July 11, 2018, Updated October 12, 2021