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



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


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.
Journal of Physical Chemistry C


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


Dimitrievska, M. , Chotard, J. , Janot, R. , Faraone, A. , Tang, W. , Skripov, A. and Udovic, T. (2018), Tracking the Progression of Anion Reorientational Behavior between &#945;-Phase and &#946;-Phase Alkali-Metal Silanides, MSIH<sub>3</sub>, by Quasielastic Neutron Scattering, Journal of Physical Chemistry C, [online], (Accessed May 29, 2024)


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Created October 24, 2018, Updated October 12, 2021