Freezing of the tetrahedral amorphous network in supercooled water triggers crystallization towards LDA ice

Abstract

In this work, we provide mechanistic insight into the initial stages of formation of ice across the limit of stability of supercooled water. Such an analysis is particularly important since crystal nucleation is not a relevant mechanism under these conditions. Using molecular dynamics simulation with the TIP4P/2005 potential, water is cooled at a constant pressure with cooling rates of 5 to 10 K per nanosecond. As the liquid is cooled across the temperature of maximum density (T_0 = 277 K), we find that there is a continuous increase in the tetrahedrality of the system. As the cooling continues across the limit of stability of water (T_s $\approx$ 235 K), large scale thermal fluctuations dissipate while the thermal equilibration is achieved through small scale fluctuations. This phenomenon, known as the dynamical crossover [Goutam et. al. in J. Stat. Phys., 168: 1302--1318 (2017)], ends the existence of the liquid state. Subsequently, we find that the tetrahedral network drives the decrease of energy and density. This process terminates when the network undergoes `freezing' (i.e., the bonds of the network acquire sufficient rigidity), due to which the network evolution, as a whole, stops. This triggers a qualitative change in the relaxation mechanism: subsequent relaxation occurs through crystallization, i.e., an increase in the structural order. In particular, we find that the cubic and hexagonal crystalline motifs, which possess medium range order, increase rapidly across the freezing point. In the resulting LDA ice states, cubic ice is found to have a significant contribution in the overall extent of crystallization, which is consistent with the experimental findings. Overall, our work provides the specific mechanism by which crystallization (leading to LDA ice) is initiated across the limit of stability of supercooled water.