Livermore researchers have performed ab initio calculations that explain the unusual melting behavior in dense sodium that was observed in previous experiments. The results of the calculations show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. The researchers found that as pressure increases, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 gigapascals (GPa), accompanied by a surprisingly large, threefold drop in electrical conductivity.
To investigate the structural and electronic changes in compressed sodium, the researchers carried out a series of first-principles molecular dynamics (FPMD) simulations for pressures between 5 and 120 GPa, and temperatures up to 1,500 kelvins. To compute the melting curve over this pressure range, they used the ‘heat-until-it-melts’ approach. With this method, the melting temperatures are deduced from a series of calculations, each done at progressively higher temperature, on the three solid structures of sodium known to be stable at room temperature up to a pressure of 130 GPa. The calculations confirmed the unprecedented pressure-induced drop in the melting temperature of sodium from 1,000 kelvins at 30 GPa down to room temperature (300 kelvins) at 120 GPa, which has been observed in previous experiments.
To understand the unusual melting curve of sodium, the researchers initially focused on a sequence of solid-phase transitions with increasing pressure corresponding to changes from less-compact to more-compact atomic structures. Generally, molten metals exhibit local orders similar to those of their crystalline solid-phases, thus liquid sodium may exhibit changes in the local order of atoms along the melting curve similar to that associated with the solid-phase transitions. To examine this possibility, the researchers determined from the calculations the distribution of atoms in the second coordination shell as a function of pressure and temperature. They found that the second coordination shell of the heated face-centered cubic (fcc) solid is contracted compared to that of the heated body-centered cubic (bcc) phase, in exactly the same way as the calculations predicted for the liquid between 0 and 60 GPa. This result showed that the changes in the liquid structure, which are noticeable in the second coordination shell, correlate with the unusual shape of the melting curve, and that these changes are characteristic of a transition from bcc to fcc local order at a finite temperature. At pressures above 60 GPa, the researchers observed another change in the atomic structure of liquid sodium corresponding to a local order that bears similarity to a distorted bcc-like structure (cl16 in the crystalline phase).
The researchers also used the FPMD simulations to investigate changes in the electronic properties of dense sodium that correlate with the changes in atomic structure observed with increasing pressure and temperature. For example, they found a dramatic drop, by a factor of three, in the electrical conductivity between 40 and 80 GPa,the pressure range the pressure range where the liquid structure becomes distorted bcc-like. These results, which were published in the September 27, 2007, issue of Nature, will stimulate future experiments to verify the predictions of the simulations for molten sodium and to search for similar behavior in other dense liquid metals.