Common microscopic structural origin for water's thermodynamic and dynamic anomalies

Area of Science:

• Physical Chemistry
• Condensed Matter Physics
• Materials Science

Background:

• Water exhibits numerous anomalies, properties deviating from typical liquid behavior, with origins debated.
• Current explanations for thermodynamic and dynamic anomalies often differ, lacking a unified microscopic perspective.

Purpose of the Study:

• To provide a unified microscopic physical picture of water's anomalies.
• To explain both thermodynamic and dynamic anomalies using the concept of locally favored structures.
• To develop and validate a hierarchical model for water's anomalous behavior.

Main Methods:

• Identification of locally favored structures using a microscopic structural descriptor measuring local translational order.
• Development of a novel hierarchical two-state model.
• Extensive simulations of two popular water models.

Main Results:

• Thermodynamic anomaly strength is directly proportional to the amount of locally favored structures.
• Dynamic properties depend on local and nearest-neighbor structures.
• The hierarchical two-state model accurately explains thermodynamic and kinetic anomalies based on temperature and pressure dependence of the structural descriptor.
• Three unique predictions for supercooled water were made and verified: an Arrhenius-to-Arrhenius crossover, a maximum in dynamic heterogeneity, and a violation of the Stokes-Einstein-Debye relation at ~2Tg.

Conclusions:

• Locally favored structures provide a unified explanation for water's thermodynamic and dynamic anomalies.
• The hierarchical two-state model successfully captures water's complex behavior.
• The findings suggest that similar scenarios may explain anomalies in other liquids that form locally favored structures, including silicon, germanium, silica, and certain metallic and chalcogenide liquids.