Structural relaxation and crystallization in supercooled water from 170 to 260 K.

Abstract

The origin of water's anomalous properties has been debated for decades. Resolution of the problem is hindered by a lack of experimental data in a crucial region of temperatures, <i>T</i>, and pressures where supercooled water rapidly crystallizes-a region often referred to as "no man's land." A recently developed technique where water is heated and cooled at rates greater than 10⁹ K/s now enables experiments in this region. Here, it is used to investigate the structural relaxation and crystallization of deeply supercooled water for 170 K < <i>T</i> < 260 K. Water's relaxation toward a new equilibrium structure depends on its initial structure with hyperquenched glassy water (HQW) typically relaxing more quickly than low-density amorphous solid water (LDA). For HQW and <i>T</i> > 230 K, simple exponential relaxation kinetics is observed. For HQW at lower temperatures, increasingly nonexponential relaxation is observed, which is consistent with the dynamics expected on a rough potential energy landscape. For LDA, approximately exponential relaxation is observed for <i>T</i> > 230 K and <i>T</i> < 200 K, with nonexponential relaxation only at intermediate temperatures. At all temperatures, water's structure can be reproduced by a linear combination of two, local structural motifs, and we show that a simple model accounts for the complex kinetics within this context. The relaxation time, <i>τ</i> _<i>rel</i> , is always shorter than the crystallization time, <i>τ</i> _<i>xtal</i> For HQW, the ratio, <i>τ</i> _<i>xtal</i> /<i>τ</i> _<i>rel</i> , goes through a minimum at ∼198 K where the ratio is about 60.