Tracking the Progression of Anion Reorientational Behavior between α-Phase and β-Phase Alkali-Metal Silanides, MSiH3, by Quasielastic Neutron Scattering

Dimitrievska, Mirjana; Chotard, Jean-Noël; Janot, Raphaël; Faraone, Antonio; Tang, Wan Si; Skripov, Alexander V.; Udovic, Terrence J.

doi:10.1021/acs.jpcc.8b08257

Title: Tracking the Progression of Anion Reorientational Behavior between α-Phase and β-Phase Alkali-Metal Silanides, MSiH₃, by Quasielastic Neutron Scattering

Journal Article · Mon Oct 01 00:00:00 EDT 2018 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/acs.jpcc.8b08257· OSTI ID:1482794

Dimitrievska, Mirjana ^[1]; Chotard, Jean-Noël ^[2]; Janot, Raphaël ^[2]; Faraone, Antonio ^[3]; Tang, Wan Si ^[4]; Skripov, Alexander V. ^[5]; Udovic, Terrence J. ^[3]

National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States); National Renewable Energy Lab. (NREL), Golden, CO (United States)
Univ. of Picardie Jules Verne (France)
National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States)
National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States); Univ. of Maryland, College Park, MD (United States); Carnegie Inst. of Washington, Washington, DC (United States)
Russian Academy of Sciences (RAS), Ekaterinburg (Russian Federation). Ural Branch

Quasielastic neutron scattering (QENS) measurements over a wide range of energy resolutions were used to probe the reorientational behavior of the pyramidal SiH₃^- anions in the monoalkali silanides (MSiH₃, where M = K, Rb, and Cs) within the low-temperature ordered β-phases, and for CsSiH₃, the high-temperature disordered α-phase and intervening hysteretic transition region. Maximum jump frequencies of the β-phase anions near the β-α transitions range from around 10⁹ s^-1 for β-KSiH₃ to 10¹⁰ s^-1 and higher for β-RbSiH₃ and β-CsSiH₃. The β-phase anions undergo uniaxial 3-fold rotational jumps around the anion quasi-C₃ symmetry axis. CsSiH₃ was the focus of further studies to map out the evolving anion dynamical behavior at temperatures above the β-phase region. As in α-KSiH₃ and α-RbSiH₃, the highly mobile anions (with reorientational jump frequencies approaching and exceeding 10¹² s^-1) in the disordered α-CsSiH₃ are all adequately modeled by H jumps between 24 different locations distributed radially around the anion center of gravity, although even higher anion reorientational disorder cannot be ruled out. QENS data for CsSiH₃ in the transition region between the α- and β-phases corroborated the presence of dynamically distinct intermediate (i-) phase. The SiH₃^- anions within i-phase appear to undergo uniaxial small-angular-jump reorientations that are more akin to the lower-dimensional β-phase anion motions rather than to the multidimensional α-phase anion motions. Moreover, they possess orientational mobilities that are an order-of-magnitude lower than those for α-phase anions but also an order-of-magnitude higher than those for β-phase anions. Combined QENS and neutron powder diffraction results strongly suggest that this i-phase is associated chiefly with the more short-range-ordered, nanocrystalline portions (invisible to diffraction) that appear to dominate the CsSiH₃.