Room-temperature liquid sodium

Room-temperature liquid sodium can exist at high pressure. Melting generally occurs when the thermal agitation of atoms or molecules in a solid overpowers the attractive interactions among them. Pressure applied to a solid sample usually helps negate thermal agitation: The melting temperature customarily goes up with pressure. However, in a few materials, such as water, the melting temperature can drop on compression. The most dramatic such negative melting curve yet seen has been studied by scientists at the Carnegie Institution of Washington who looked at one of the simplest metals known—sodium. At atmospheric pressure (0.1 MPa), sodium melts at 371 K. As pressure goes up, so does the melting temperature, as high as 1000 K at a pressure of 30 GPa. Then strange things begin to happen. As the pressure rises further, the melting temperature starts to drop, and reaches a low of 300 K at 118 GPa. Between 65 and 80 GPa, the solid changes structure, from bcc to fcc. At those pressures, the sodium’s liquid phase is denser than the solid, like water and ice. Above 100 GPa, sodium begins to crystallize in denser and unexpectedly complex structures. (E. Gregoryanz et al., Phys. Rev. Lett. 94, 185502, 2005 http://dx.doi.org/10.1103/PhysRevLett.94.185502 .)