Dimitrievska Mirjana, Chotard Jean-Noël, Janot Raphaël, Faraone Antonio, Tang Wan Si, Skripov Alexander V, Udovic Terrence J
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States.
National Renewable Energy Laboratory, Golden, Colorado 80401, United States.
J Phys Chem C Nanomater Interfaces. 2019;122(42). doi: 10.1021/acs.jpcc.8b08257.
Quasielastic neutron scattering (QENS) measurements over a wide range of energy resolutions were used to probe the reorientational behavior of the pyramidal SiH anions in the monoalkali silanides (MSiH, where M = K, Rb, and Cs) within the low-temperature ordered -phases, and for CsSiH, the high-temperature disordered -phase and intervening hysteretic transition region. Maximum jump frequencies of the -phase anions near the - transitions range from around 10 s for -KSiH to 10 s and higher for -RbSiH and -CsSiH. The -phase anions undergo uniaxial 3-fold rotational jumps around the anion quasi- symmetry axis. CsSiH was the focus of further studies to map out the evolving anion dynamical behavior at temperatures above the -phase region. As in -KSiH and -RbSiH, the highly mobile anions (with reorientational jump frequencies approaching and exceeding 10 s) in the disordered -CsSiH 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 CsSiH in the transition region between the - and -phases corroborated the presence of dynamically distinct intermediate (-) phase. The SiH anions within -phase appear to undergo uniaxial small-angular-jump reorientations that are more akin to the lower-dimensional -phase anion motions rather than to the multidimensional -phase anion motions. Moreover, they possess orientational mobilities that are an order-of-magnitude lower than those for -phase anions but also an order-of-magnitude higher than those for -phase anions. Combined QENS and neutron powder diffraction results strongly suggest that this -phase is associated chiefly with the more short-range-ordered, nanocrystalline portions (invisible to diffraction) that appear to dominate the CsSiH.
在很宽的能量分辨率范围内进行了准弹性中子散射（QENS）测量，以探究低温有序相中单碱硅化物（MSiH，其中M = K、Rb和Cs）中金字塔形SiH阴离子的重排行为，对于CsSiH，还研究了高温无序相和中间的滞后转变区域。在β转变附近，β相阴离子的最大跳跃频率范围从-KSiH的约10¹¹ s⁻¹到-RbSiH和-CsSiH的10¹² s⁻¹及更高。β相阴离子围绕阴离子准三重对称轴进行单轴3重旋转跳跃。CsSiH是进一步研究的重点，以描绘出高于β相区域温度下阴离子动态行为的演变。与-KSiH和-RbSiH一样，无序的β-CsSiH中高度移动的阴离子（重排跳跃频率接近并超过10¹² s⁻¹）都可以通过在围绕阴离子重心径向分布的24个不同位置之间的H跳跃得到充分建模，尽管不能排除更高的阴离子重排无序性。CsSiH在β相和γ相之间的转变区域的QENS数据证实了动态不同的中间（β-γ）相的存在。γ相内的SiH阴离子似乎经历单轴小角度跳跃重排，这更类似于低维β相阴离子运动，而不是多维γ相阴离子运动。此外，它们的取向迁移率比β相阴离子低一个数量级，但比α相阴离子高一个数量级。QENS和中子粉末衍射结果相结合强烈表明，这个β-γ相主要与似乎在CsSiH中占主导地位的更短程有序的纳米晶部分（衍射不可见）相关。
