Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan and The Graduate University for Advanced Studies, Myodaiji, Okazaki, Aichi 444-8585, Japan.
Indian Institute of Science, Bangalore 560012, India.
J Chem Phys. 2018 Sep 28;149(12):124504. doi: 10.1063/1.5044458.
Properties of water have been well elucidated for temperatures above ∼230 K and yet mysteries remain in the deeply supercooled region. By performing extensive molecular dynamics simulations on this supercooled region, we find that structural and dynamical instabilities are hidden in the experimentally inaccessible region between 235 K and 150 K. We find a hitherto undiscovered fragmentation from 220 K to 190 K, which is the breakup of large clusters consisting of molecules with a locally distorted tetrahedral structure into small pieces with one or two isolated defects. The fragmentation leads to considerable changes in the relaxation dynamics of water. We reveal a crucial role of specific three-coordinated defects in slow but persistent structural relaxation. The presence of relaxation due to these specific defects makes the water glass transition temperature (=136 K) extremely low and explains why the of water is ∼1/2 of the melting temperature , much lower than the commonly obeyed 2/3 rule of / .
水的性质在高于 ∼230 K 的温度下已经得到了很好的阐明，但在深度过冷区域仍存在一些谜团。通过对这个过冷区域进行广泛的分子动力学模拟，我们发现结构和动力学不稳定性隐藏在实验上无法到达的 235 K 和 150 K 之间的区域。我们发现了一个迄今为止未被发现的从 220 K 到 190 K 的片段，即由具有局部扭曲四面体结构的分子组成的大团簇分裂成具有一个或两个孤立缺陷的小块。这种碎裂导致了水的弛豫动力学的显著变化。我们揭示了特定的三配位缺陷在缓慢但持续的结构弛豫中起着关键作用。由于这些特定缺陷的存在，使得水的玻璃化转变温度 Tg（=136 K）极低，解释了为什么水的 Tg 约为熔点 Tm 的 1/2，远低于通常遵循的 2/3 规则 / 。
