Room Temperature Liquid Sodium

Room temperature liquid sodium can occur but only under pressures of a million atmospheres. Melting is a mystery. It happens when the thermal agitation among atoms in a solid overcomes the inter-atom bonds. Applying pressure to a solid sample usually helps to negate the effect of thermal agitation and so the melting temperature usually goes up with pressure. In a few materials, such as water, above a certain pressure the melting point begins to drop.

Now, the most dramatic case yet seen of such a “negative melting curve” has been studied by scientists at the Carnegie Institution of Washington looking at one of the simplest metals known, sodium. What happens is this: With zero pressure applied, sodium melts at a temperature of 371 K. As pressure is added, the melting temperature goes up too, up to 1000 K at a pressure of 30 giga-pascals (30 GPa), or about 300,000 atm.

Then strange things happen. As the pressure is taken up further, the melting point starts to drop, reaching a low of 300 K (below its ambient melting point) at pressures of 118 GPa (see graph at www.aip.org/png). All previous materials exhibiting negative melting curves have gone negative very reluctantly, over pressure ranges of a few GPa or temperature ranges of a few K. Sodium, by contrast, goes negative over a range of 700 K and 80 GPa.

According to Carnegie researcher Eugene Gregoryanz (e.gregoryanz@gl.ciw.edu), at a pressure of a million atmospheres his sodium sample melts at room temperature. The liquid is denser than the solid (water shares this trait), and might have strange plastic or mechanical properties. It might even be superconducting under some circumstances, he says. (Gregoryanz et al., Physical Review Letters, upcoming article.