Unexpected observation of ice at low temperature, high pressure questions water theory

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Toronto, May 23 (Canadian-Media): Through an experiment designed to create a super-cold state of water, scientists at the Department of Energy's Oak Ridge National Laboratory, the largest US Department of Energy science and energy laboratory, used neutron scattering to discover a pathway to the unexpected formation of dense, crystalline phases of ice thought to exist beyond Earth's limits, Science X Newsletter reports said yesterday.

Scientists at Oak Ridge National Laboratory studying super-cold states of water discovered a pathway to the unexpected formation of dense, crystalline phases of ice thought to exist beyond Earth's limits. Their findings, reported in Nature, challenge accepted theories and could lead to better understanding of ice found on other planets, moons and elsewhere in space. Credit: Jill Hemman/Oak Ridge National Laboratory, US Dept. of Energy

Observation of these particular crystalline ice phases, known as ice IX, ice XV and ice VIII, challenges accepted theories about super-cooled water and amorphous, or non-crystalline, ice. The researchers' findings, reported in the journal Nature, will also lead to better basic understanding of ice and its various phases found on other planets and moons and elsewhere in space.

"Hydrogen and oxygen are among the most abundant elements in the universe, and the simplest molecular compound of the two, H2O, is common," said Chris Tulk, ORNL neutron scattering scientist and lead author. "In fact, a popular theory suggests that most of Earth's water was brought here through collisions with icy comets."​

On Earth, when water molecules reach zero degrees Celsius, they enter a lower energy state and settle onto a hexagonal crystalline lattice. This frozen form is denoted as ice Ih, the most common phase of water that can be found in household freezers or at skating rinks.

Ice IX, ice XV and ice VIII are three of at least 17 ice phases realized when molecules reorganize into a stable crystalline structure at varying super-low temperatures and very high pressures, conditions that don't occur naturally on Earth.

"As ice changes phases, it's similar to water going from a gas to a liquid to a solid except at low temperatures and high pressure—the ice transforms between various different solid forms," Tulk said.

Each known ice phase is characterized by its unique crystal structure within its pressure-temperature range of stability, where the molecules reach equilibrium and the water molecules exhibit a regular three-dimensional pattern, and the structure becomes stable.

Initially, Tulk and colleagues at the National Research Council of Canada and from the University of California at Los Angeles were exploring the structural nature of amorphous ice—a state of ice that forms with no ordered crystalline structure—as it recrystallizes at even higher pressures.