The glass structure of a material is often believed to mimic its corresponding liquid. Polyamorphism between ices has been used as a guide to elucidate the properties of liquid water. But how many forms of amorphous ice cream are there? Do we understand how high pressure metastable crystalline ice evolves into the thermally stable low density form? An international research team led by Chuanlong Lin and Wenge Yang of HPSTAR and John S. Tse of the University of Saskatchewan has revealed a multi-step transformation mechanism that uses state-of-the-art synchrotron X-ray diffraction with temporal resolution. A temperature / time dependent kinetic pathway was identified with three distinctive transitions in the structural evolution from metastable crystalline ice (ice VII or ice VIII) to thermodynamically stable ice I. These intermediate processes compete with each other. The end result is a juxtaposition of these processes. The work is published in PNAS.
Water plays a vital role in the origin of life on Earth. In the liquid phase, it exhibits many unusual properties. In the solid phase, ordinary ice also shows various phase transitions at high pressure. Many theoretical and experimental studies have been devoted to understanding the underlying mechanisms of conversion. Until now, most of the experiments have been ex situ measurements on recovered samples and lack detailed information on the structural evolution that accompanies the transformation. Previous studies have been hampered by technical difficulties in monitoring rapid structural change over a wide range of pressure and temperature.
An illustration shows the structural evolution of ice VII as a function of time under constant PT conditions. Credit: Chuanlong Lin
In 2017, Lin and his colleagues overcame the experimental challenge. A series of studies was conducted to investigate ice transitions by combining time-resolved X-ray diffraction in situ and remote pressure control with different ramp rates within a low-temperature cryostat. This ability allowed the suppression of thermally driven crystalline-crystalline transitions[[[[PNAS 115, 2010-2015 (2018)]. Important insights were gained on the complexity of poly-amorphous transformations, such as kinetically controlled two-step amorphization on ice Ih [Phys. Rev. Lett. 119, 135701(2017)] and the successful adventure in no man’s land [Phys. Rev. Lett. 121, 225703(2018)].
Now, they try to answer what exactly is the nature of amorphous-amorphous phase transformation processes? Using recently developed techniques, they explored the “mirror” process, that is, the inverse transformation of a metastable high-density crystalline ice (ie Ice VII or Ice VIII) to stable ice at room temperature I. They identified the temperature / time-dependent kinetic pathways and characterized the interaction / competition between the high-density amorphous transition (HDA) and the low-density amorphous transition (LDA) and recrystallization. Unlike the previously reported Ice VII (or Ice VIII) – LDA – Ice I transformation sequences, time-resolved measurements show a three-step process: initial transformation from Ice VII to HDA, followed by an HDA transition – LDA, and then crystallization of LDA on ice I. Both the amorphization of ice VII and the transition from HDA to LDA show distinctive mechanisms of thermal activation. Significantly, both processes exhibit Arrhenius behavior with a temperature dependent duration time (τ) and a ‘transition’ temperature of around 110-115 K.
Large-scale molecular dynamics calculations also support his experimental findings. Furthermore, it shows that the transformation from HDA to LDA is continuous with a large difference in density and involves substantial water shifts at the nanoscale. This study presents a new perspective on metastability and complexities in the configuration of kinetic ice transition routes.
Reference: “Temperature-dependent kinetic pathways presenting distinctive thermal activation mechanisms in the structural evolution of ice VII” by Chuanlong Lin, Xuqiang Liu, Xue Yong, See ORCID profile John S. Tse, Jesse S. Smith, Niall J. English, Bihan Wang, Mei Li, Wenge Yang, and Ho-Kwang Mao, June 22, 2020, procedures of the National Academy of Sciences.
DOI: 10.1073 / pnas.2007959117
