Solid to Liquid to Gas? Reconstructive Phase Transitions Don't Care About Your Rules
In 2017, researchers announced that they successfully melted a solid below the temperature of its freezing point in a brilliant demonstration of something called a reconstructive phase transition. Confused? We'll explain.
A Different Phase Flavor
You're likely familiar with the idea that a solid melts into a liquid and a liquid vaporizes into a gas. Those are examples of simple phase transitions. They happen when chemical bonds in a substance morph and rearrange while staying intact. But there's another "flavor" of phase transition you may not have learned in school. That one is called a reconstructive phase transition, and it's much more unpredictable—and way more bizarre. Reconstructive phase transitions involve breaking and rearranging parts of chemical bonds into new structures. And, surprise! Those phase transitions are the most common ones in nature.
What The Researchers Accomplished
As they announced in a 2017 paper published in the journal Nature, researchers at the Carnegie Institution for Science successfully melted the crystalline (or, technically, frozen) form of the metal bismuth into a liquid without raising its temperature. Here's how: they placed it in a diamond anvil cell and subjected it to intense pressures and decompression while keeping it at a constant temperature of 216ºC (420ºF or 489 K)—a few degrees colder than its melting point of 271.52ºC. The pressures ranged from 32,000 times atmospheric pressure, or "atmospheres," to 12,000 atmospheres. In the process of depressurizing, they hit the sweet spot: at around 23,000 atmospheres, the bismuth melted into a liquid state, with no heat required. Once it was depressurized to 12,000 atmospheres, it recrystallized back into a solid.
When the bismuth melted, it turned into a particular kind of substance known as a "metastable liquid." That is, a liquid that is stable if you leave it alone—this particular liquid was able to stay as-is for hours, as long as the pressure remained constant—but immediately reverts back to its solid form if it's disturbed. As Gizmodo put it, "Metastable liquids have found a little nook in the laws of physics where they can stay liquid-like—but like a spinning plate balancing on a stick, any perturbation and the atoms zip back into a solid form."
This tricky science could have wide-reaching applications. According to the study's press release, it could be important "for developing new materials and for understanding the dynamics of planetary interiors, such as earthquakes, because a metastable liquid could act as a lubricant strongly affecting the dynamics of the Earth's interior."