4.6 Article

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

Journal

JOURNAL OF PHYSICAL CHEMISTRY C
Volume 122, Issue 27, Pages 15198-15207

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcc.8b04385

Keywords

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Funding

  1. US DOE Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office [DE-AC36-08GO28308]
  2. Russian Federal Agency of Scientific Organizations (program Spin) [AAAA-A18-118020290129-5]
  3. U.S. Department of Energy's National Nuclear Security Administration [DE-NA-0003525]
  4. National Science Foundation [DMR-0944772, DMR-1508249]
  5. US DOE [DE-AC02-06CH11357]

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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,:approximate to 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 C-4 symmetry axis (e.g., with correlation frequencies of approximate to 2.6 X 1010 s(-1) at 530 K) combined with order of magnitude slower orthogonal 180 reorientational flips (e.g., X 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 C-4 symmetry axis to preserve the ordered (RbB10H10)-B-2 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) and 288(3) meV for the C4 fourfold jumps and apical exchanges, respectively, between 400 and 680 K. Below this temperature range, NMR (and QENS) both indicate a shift to significantly larger reorientational barriers, for example, 485(8) meV for the apical exchanges. Finally, subambient diffraction measurements identify a subtle change in the Rb2B10H10, structure from monoclinic to triclinic symmetry as the temperature is decreased from around 250 to 210 K.

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