Tracking the Progression of Anion Reorientational Behavior between α-Phase and β-Phase Alkali-Metal Silanides, MSIH3, by Quasielastic Neutron Scattering

Published

October 25, 2018

Author(s)

Mirjana NMN Dimitrievska, Jean-Noel Chotard, Raphael Janot, Antonio Faraone, Wan Si Tang, Alexander V. Skripov, Terrence J. Udovic

Abstract

Quasielastic neutron scattering (QENS) measurements over a wide range of resolutions were used to probe the reorientational behavior of the pyramidal SiH₃- anions in the mono-alkali silanides (MSiH₃, where M-K, Rb, and Cs) within the low-temperature ordered β-phases, high-temperature disordered α-phases, and intervening hysteretic transition regions. Maximum jump frequencies of the β-phase anions near the β-α transitions range from around 10⁹ s^-1 for β-KSiH₃ to 10¹⁰s^-1 and higher for βRBSiH₃ and β-CsSiH₃. The β-phase anions undergo uniaxial three-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 regions. 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 both CsSiH₃ and RbSiH₃ in the transition nanosized, intermediate (i-) phases, as suggested previously. The SiH₃- anions within these nucleated i-phases appear to undergo uniaxial small-angular-jump reorientations that are more akin to the lower-dimensional β-phase anion motions rather than to the multi-dimensional α-phase anion motions. Moreover, they possess orientational mobilities that are an order-of-magnitude lower than those of α-phase anions but also an order-of-magnitude higher than thos for β-phase anions.

Citation

Journal of Physical Chemistry C

Volume

122

Issue

42

Pub Type

Journals

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Keywords

jump diffusion, neutron scattering, phase transitions, quasielastic neutron scattering, reorientational mechanism, rotational motion, silanides, silanyl anion, spin-echo spectroscopy

Citation

Dimitrievska, M. , Chotard, J. , Janot, R. , Faraone, A. , Tang, W. , Skripov, A. and Udovic, T. (2018), Tracking the Progression of Anion Reorientational Behavior between α-Phase and β-Phase Alkali-Metal Silanides, MSIH₃, by Quasielastic Neutron Scattering, Journal of Physical Chemistry C, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=926061 (Accessed October 2, 2025)