Metal Rebel: Under extreme pressure, sodium breaks the rules for turning into liquid

In a demonstration that defies basic assumptions in physics, researchers have created liquid sodium at room temperature.

As a member of the alkali metals—the first column in the periodic table—sodium is considered to be one of the simplest metals. Under normal conditions, it melts at about 98°C, just under water’s boiling point. The new observation that sodium can melt at just above room temperature, 27°C, confirms a growing suspicion among scientists that alkali metals are more complicated than they previously thought.

Eugene Gregoryanz of the Carnegie Institution of Washington (D.C.) and his colleagues loaded a microscopic sample of solid sodium into a diamond-anvil cell, a viselike device capable of generating extreme pressure.

The researchers repeatedly raised the pressure inside the cell and determined sodium’s melting temperature at each of these pressures by heating the material.

When a solid is squeezed under high pressure, its constituent atoms or molecules pack together tightly and have less room to move about. Under such conditions, it takes more thermal energy than normal to increase the motion of the atoms or molecules enough to cause the solid to melt. Thus, increasing pressure typically raises a material’s melting temperature.

However, when Gregoryanz and his colleagues raised the pressure on solid sodium beyond 30 gigapascals—300,000 times atmospheric pressure at sea level—the metal’s melting temperature began to drop. In fact, its melting temperature continued to drop until the pressure reached about 100 gigapascals, at which point the sample melted at near-room temperature. The researchers report their findings in an upcoming Physical Review Letters.

Water behaves in this same unusual way, but over only a narrow pressure range. Between normal atmospheric pressure and 1,000 atmospheres, which is about the pressure of the deep ocean, liquid water is denser than ice, which explains why ice floats, says Gregoryanz. Similarly, liquid sodium under high pressure is denser than its solid form.

“Sodium really departs from its simplicity in extraordinary ways,” says Neil Ashcroft of Cornell University. Like all alkali metals, sodium has one free electron that can react with other elements. Normally, this electron is far from the atom’s 10 other electrons, which reside closer to sodium’s nucleus. Squeezing the metal forces the free electron closer to the others. Gregoryanz speculates that this forced liaison among electrons might explain sodium’s strange behavior.

During the experiment, the researchers noted that solid sodium under high pressure assumed several odd crystal structures, suggesting that the liquid itself could have unusual properties. “It might be superfluid; it might be superconducting,” says Gregoryanz.

Previous studies on other alkali metals, including lithium and hydrogen, have shown that these elements also behave strangely at high pressures and can become superconducting. “We all thought there was nothing much left to say about the alkali metals because they’re all so simple. But as soon as you squeeze them up, the physics becomes extremely interesting,” says Ashcroft.