Nature of Decahydro-closo-decaborate Anion Reorientations in an Ordered Alkali-Metal Salt: Rb2B10H10
[1];
[2];
[3];
[4];
[3];
[4]
- National Renewable Energy Lab. (NREL), Golden, CO (United States); National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States). Center for Neutron Research
- Sandia National Lab. (SNL-CA), Livermore, CA (United States)
- Russian Academy of Sciences (RAS), Ekaterinburg (Russian Federation)
- National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States). Center for Neutron Research
- National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States). Center for Neutron Research; Univ. of Maryland, College Park, MD (United States); Carnegie Inst. of Washington, Argonne, IL (United States). Geophysical Lab.
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 degree jumps around the anion C4 symmetry axis (e.g., with correlation frequencies of ~2.6 x 1010 s-1 at 530 K) combined with order of magnitude slower orthogonal 180 degree reorientational flips (e.g., ~3.1 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 degree 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) 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.
- Research Organization:
- Sandia National Lab. (SNL-CA), Livermore, CA (United States); National Renewable Energy Lab. (NREL), Golden, CO (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA); USDOE Office of Energy Efficiency and Renewable Energy (EERE), Sustainable Transportation Office. Hydrogen Fuel Cell Technologies Office
- Grant/Contract Number:
- AC04-94AL85000; NA0003525; DMR-0944772; DMR-1508249; AC02-06CH1135; AC36-08GO28308
- OSTI ID:
- 1507407
- Alternate ID(s):
- OSTI ID: 1476875
- Report Number(s):
- SAND-2019-3247J; NREL/JA-5900-72563; 673701
- Journal Information:
- Journal of Physical Chemistry. C, Vol. 122, Issue 27; ISSN 1932-7447
- Publisher:
- American Chemical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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