Compiling the cold hard facts about imperfections in ice

The form of ice sitting in your freezer, known as hexagonal ice or ice I_h, plays a role in many processes on Earth, including glacier flow and lightning strikes. Structural defects in the ice crystals can affect these processes, but the relationships between the many different kinds of defects and their effects on ice crystal properties are not fully understood.

In this Perspective, Maurice de Koning summarizes 50 years of findings on defects in hexagonal ice crystals, hoping to provide a reference on ice crystal imperfections and their effects on crystal properties.

De Koning discusses zero-dimensional imperfections, such as point defects where one molecule is missing from a crystal; one-dimensional imperfections, such as line defects and dislocations; and two-dimensional imperfections. A thorough understanding of the various crystal imperfections in hexagonal ice may help explain defects in other crystalline phases of water.

He also presents future directions and persisting challenges in the field. For instance, the added complexity in ice studies due to the molecular nature of ice, which adds an extra degree of freedom regarding the molecule’s orientation compared to atomic crystals.

“It has been difficult to achieve as much understanding as for materials where the orientational degree of freedom is not there,” said de Koning. “There are many other things that are still unresolved. There are many things to be done.”

Another subject of interest is a type of 2D defect known as grain boundaries, which occur at the plane between two crystal grains. A better understanding of the mechanism behind grain boundary sliding – when the two grains slide against each other – could be useful in understanding glacier flow.

Source: “Crystal imperfections in ice I_h,” by Maurice de Koning, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0019067 .