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

Published

July 12, 2018

Author(s)

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

Abstract

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 B₁₀H₁₀^2- 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.

Citation

Journal of Physical Chemistry C

Volume

122

Issue

27

Pub Type

Journals

Download Paper

Local Download

Keywords

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

Citation

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: Rb₂B₁₀H₁₀, Journal of Physical Chemistry C, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=926004 (Accessed December 12, 2024)