Diffusive dynamics during the high-to-low density transition in amorphous ice
Diffusive dynamics during the high-to-low density transition in amorphous ice
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DOI：
10.1073/pnas.1705303114
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发表时间：
2017-08-01
影响因子：
11.1
通讯作者：
Nilsson, Anders
中科院分区：
文献类型：
--
作者：
Perakis, Fivos;Amann-Winkel, Katrin;Nilsson, Anders
水以高密度和低密度无定形冰(HDA和LDA)的形式存在，对应于相图亚稳部分的高密度(HDA)和低密度液体(LDL)的玻璃态。然而，玻璃化转变和从高密度到低密度转变的性质都存在争议，还需要新的实验证据。在此，我们将广角X射线散射(WAXS)和X射线光子相关谱(XPCS)相结合，在小角X射线散射(SAXS)几何结构中探索了1bar无定形冰从高密度到低密度转变过程中的结构和动力学性质。通过分析结构因子和径向分布函数，在T=125K时观察到两个结构不同的磁区共存，XPCS探索了动量空间的动力学，这在SAXS几何结构中反映了纳米尺度上的结构弛豫。HDA的动力学特征是由粘弹性松弛和纳米非均质释放的应力引起的具有大时间常数的慢分量。在110K以上，出现了一个更快的、强烈依赖于温度的分量，动量转移依赖于纳米尺度的扩散。这种动力学成分在130K转变为低密度形式后减慢，但仍是扩散的。在不同的解释下，讨论了高密度和低密度形式的扩散特性，其结果与超粘性区域的液-液转变假说最为一致。
Water exists in high- and low-density amorphous ice forms (HDA and LDA), which could correspond to the glassy states of high(HDL) and low-density liquid (LDL) in the metastable part of the phase diagram. However, the nature of both the glass transition and the high-to-low-density transition are debated and new experimental evidence is needed. Here we combine wide-angle X-ray scattering (WAXS) with X-ray photon-correlation spectroscopy (XPCS) in the small-angle X-ray scattering (SAXS) geometry to probe both the structural and dynamical properties during the high-to-low-density transition in amorphous ice at 1 bar. By analyzing the structure factor and the radial distribution function, the coexistence of two structurally distinct domains is observed at T = 125 K. XPCS probes the dynamics in momentum space, which in the SAXS geometry reflects structural relaxation on the nanometer length scale. The dynamics of HDA are characterized by a slow component with a large time constant, arising from viscoelastic relaxation and stress release from nanometer-sized heterogeneities. Above 110 K a faster, strongly temperature-dependent component appears, with momentum transfer dependence pointing toward nanoscale diffusion. This dynamical component slows down after transition into the low-density form at 130 K, but remains diffusive. The diffusive character of both the high- and low-density forms is discussed among different interpretations and the results are most consistent with the hypothesis of a liquid-liquid transition in the ultraviscous regime.