Scientists have discovered a previously unknown transitional form of crystalline ice called Ice-VIIt, using a new method of studying substances at high pressure. It is likely that water in this aggregate state can exist inside the crust and mantle of the Earth, as well as on other water-rich planets.
Ice, like other solids, is capable of taking different forms that depend on temperature and formation pressure. For example, carbon can exist in the form of diamond or graphite. Ice, on the other hand, is unique because of its large number of phases. To date, at least 20 of its forms are known: three amorphous and 17 crystalline. Now, researchers at the University of Nevada in Las Vegas (USA) have discovered an entirely new form of ice and described it in an article published in Physical Review B.
The authors of the paper conducted an experiment in a diamond anvil cell. This design consists of two conically shaped diamonds facing each other, between which a sample of the substance under investigation is placed. Due to the exceptional hardness of the diamonds, a pressure of several million atmospheres can be generated on the sample bed, which has a diameter of less than a millimetre. And the transparency of diamonds makes it possible to study the sample in a variety of ways.
However, the scientists didn't just compress the ice, they also heated it using laser light. First, a sample of water was placed between two diamonds and pressurised until it froze into several mixed ice crystals. This ice was then heated with a laser so that it had time to melt before it became a crystal again. In this way, the researchers gradually raised the pressure in the cell, periodically shooting a laser beam into the sample.
The scientists watched as the ice made the transition from the studied phase with a cubic crystal lattice Ice-VII to a previously unknown intermediate phase with a tetragonal lattice, called Ice-VIIt. After that, the sample passed into another known and also cubic Ice-X phase, which forms at higher pressures than Ice-VII. However, the formation of Ice-X was already observed at a pressure of 300 thousand atmospheres, well below a million atmospheres - the very pressure previously considered the threshold of formation of Ice-X.
According to the authors of the work, the probability of detecting Ice-VIIt on the surface of the Earth is very low, but it is possible that this form is present inside the crust and upper mantle of our planet. In addition, Ice-VIIt may exist on other water-rich planets and their large satellites outside the solar system.
