Institute of Physical Chemistry, University of Innsbruck, A-6020, Innsbruck, Austria.
Sci Rep. 2017 Jun 21;7(1):3995. doi: 10.1038/s41598-017-03583-2.
The crystallisation behaviour of very high-density amorphous ice (VHDA) and unannealed high-density amorphous ice (uHDA) has been studied in situ by volumetry and ex situ by powder x-ray diffraction in the intermediate pressure range 0.7-1.8 GPa employing different heating rates (0.5, 5 and 30 K min). This study shows that at pressures >1 GPa the crystallisation behaviour of VHDA and uHDA is basically the same for all heating rates. That is, parallel crystallisation is almost entirely suppressed with mainly ice XII forming. This contrasts former results reporting parallel crystallisation to approximately levelled phase mixtures of ice IV and ice XII even at higher pressures for uHDA. We speculate this to be due to formation of microcracks upon decompression in earlier works, but not in the present one. Crystallisation temperatures T are up to 16 K higher than previously reported, raising the low-temperature border to no man's land and opening a considerably larger window for future studies on non-crystalline water. The results indicate uHDA to contain heterogeneities on the nanoscale, but VHDA to be rather homogeneous with nano-crystallites being largely absent. Upon transforming uHDA to VHDA, the nano-scale heterogeneities disappear for >1 GPa whereas microcracks do not.
非常高浓度非晶冰（VHDA）和未经退火的高密度非晶冰（uHDA）的结晶行为在中间压力范围内（0.7-1.8 GPa）通过体积法和非原位粉末 X 射线衍射进行了原位和非原位研究，采用了不同的加热速率（0.5、5 和 30 K min）。本研究表明，在压力 >1 GPa 时，VHDA 和 uHDA 的结晶行为对于所有加热速率基本相同。也就是说，平行结晶几乎完全被抑制，主要形成冰 XII。这与以前的结果形成对比，以前的结果报告称，即使在更高的压力下，uHDA 的平行结晶也会导致冰 IV 和冰 XII 的大致平衡的相混合物。我们推测这是由于在早期的工作中减压时形成微裂纹，但在本工作中没有。结晶温度 T 比以前报道的高 16 K，将低温边界推向无人区，并为未来关于非晶态水的研究打开了一个更大的窗口。结果表明，uHDA 在纳米尺度上存在不均匀性，但 VHDA 相当均匀，几乎不存在纳米晶。在将 uHDA 转化为 VHDA 时，纳米尺度的不均匀性在>1 GPa 时消失，而微裂纹不会消失。
