Department of Fundamental Engineering, Institute of Industrial Science, University of Tokyo, Tokyo 153-8505, Japan.
Department of Fundamental Engineering, Institute of Industrial Science, University of Tokyo, Tokyo 153-8505, Japan
Proc Natl Acad Sci U S A. 2018 Feb 27;115(9):1980-1985. doi: 10.1073/pnas.1717233115. Epub 2018 Feb 8.
Water and silica are the most important materials with local tetrahedral symmetry. They have similar crystalline polymorphs and exhibit anomalous density maximum in the liquid state. However, water and silica also show very different characteristics. For instance, the density of water varies much more sharply than that of liquid silica near the maximum as temperature changes. More notably, silica is a very good glass-former, but water is an extremely poor one. The physical origins of these similarities and differences still remain elusive, due to the lack of a microscopic understanding of the structural ordering in these two important liquids. Here, by accessing microscopic structural information by computer simulations, we reveal that local translational symmetry breaking is responsible for the density anomalies. On the other hand, the difference in the degree of local orientational symmetry breaking between water and silica, which originates from the difference in their bonding nature, causes not only the difference in the sharpness of density anomalies, but also their distinct glass-forming abilities. Our work not only shows the crucial roles of local translational and orientational symmetry breaking in the physical properties of the two extremely important materials, water and silica, but also provides a unified scenario applicable for other tetrahedral liquids such as Si, Ge, C, BeF, and GeO.
水和二氧化硅是具有局部四面体对称性的最重要的物质。它们具有相似的结晶多形体，并在液态下表现出异常的密度最大值。然而，水和二氧化硅也表现出非常不同的特性。例如，随着温度的变化，水的密度变化比液态二氧化硅更急剧。更值得注意的是，二氧化硅是一种非常好的玻璃形成剂，但水则是一种极差的玻璃形成剂。由于缺乏对这两种重要液体中结构有序性的微观理解，这些相似性和差异性的物理起源仍然难以捉摸。在这里，通过计算机模拟获取微观结构信息，我们揭示了局部平移对称性的破坏是导致密度异常的原因。另一方面，水和二氧化硅之间局部取向对称性破坏程度的差异，源于它们的键合性质的差异，不仅导致了密度异常的尖锐程度的差异，而且导致了它们截然不同的成玻能力。我们的工作不仅表明了局部平移和取向对称性破坏在水和二氧化硅这两种极其重要的材料的物理性质中的关键作用，而且还为其他四面体液体（如 Si、Ge、C、BeF 和 GeO）提供了一个统一的适用情景。
