The melting temperature of sodium is unique, being lower at very high pressures than at ambient pressure. In a recent experiment at beamline ID27, this unique feature of sodium was explorated in order to conduct the highest-pressure single-crystal structure refinements ever made. This has revealed that the low melting temperature of elemental sodium is associated with a number of extremely complex new crystal structures. The complex phase diagram of this “simple” metal above 100 GPa suggests extraordinary states in both the liquid and solid phases at extreme conditions and has implications for other “simple” metals.

For decades, a combination of fundamental and applied physics has been utilised to synthesise new materials having intriguing and potentially useful properties such as super-hardness or superconductivity. When materials are compressed, their bulk and shear moduli and their strength (all measures of how compressible and therefore how hard materials are) as well as their melting temperatures all typically increase. For example, at a pressure of 55 GPa, the strength of elemental argon exceeds that of hardened steel under ambient conditions [1], while its melting temperature above 130 GPa is expected to exceed that of iron. The two lightest elements – hydrogen and helium – have extremely high compressibility, and helium, owing to its chemical inertness, is commonly used as a pressure transmitting medium to create hydrostatic pressures in high-pressure experiments. At pressures up to 25 GPa, the melting temperatures of hydrogen and helium are the lowest of all elements and lie below 500 K. However, their melting temperatures continue to increase monotonically with pressure, reaching ~1000 K at 100 GPa, while their strength at these pressures is such that they do not create even quasi-hydrostatic conditions.

Recently, it has been shown that although the melting temperature of sodium metal initially behaves “normally” and increases from 370 K at ambient pressure to ~1000 K at 30 GPa, it then decreases rapidly and drops to just above room temperature at pressures of 118 GPa, before increasing again [2]. The melting temperature at 118 GPa is thus lower than at ambient pressure – behaviour that is unique amongst all known materials. The low melting temperature of sodium at 118 GPa means it is the only known “super-soft” system which can be used as a truly hydrostatic medium at pressures above 100 GPa. High-pressure single-crystal diffraction experiments were carried out using the high intensity and micro-focused X-ray beam available at beamline ID27. The minimum in the melting curve of sodium at 118 GPa was shown to be associated with a large number of extremely complex crystal structures (Figure 33). Indeed, one of these structures contains more than 500 atoms in the unit cell.

Fig. 33: Observed phases of sodium in the vicinity of the minimum of the melting curve.

The low melting temperature of sodium at 118 GPa means that it is relatively easy to melt the sample in situ and the proximity of the melting curve ensures hydrostatic conditions. It was therefore possible to grow high quality single crystals at such high pressures by slow cooling from the liquid phase. A typical single-crystal diffraction image collected under these extreme conditions is presented in Figure 34. The number of different phases found in the vicinity of the melting minimum was surprising, seven in total. Very small changes in pressure and/or temperature resulted in transitions between the different structural forms. Several of the structures were extremely complex – more so than anything else previously observed for any element, even at ambient conditions.

Fig. 34: Composite diffraction image showing representative data from the primitive trigonal phase with 90 atoms in the unit cell.

Using single-crystal methods, which have been developed as a three-year long term project at the ESRF, it has been possible to determine the lattice parameters of all seven phases and, so far, to refine the atomic positions in three of them – reaching the highest ever pressure for which the full single-crystal structure refinement has been carried out. The composite diffraction image in Figure 34 shows representative data from a primitive trigonal structure with 90 atoms in the unit cell.

Early theoretical calculations on hydrogen have suggested that at extreme compressions it might have a liquid ground state with very unusual properties and a family of the related anisotropic structures associated with its liquid state [3]. The results on sodium show something very similar being realised in nature, giving hope that the theoretically-predicted bizarre states of hydrogen do indeed exist even though they are currently unreachable in experimental studies.

 

Principal publication and authors

E. Gregoryanz (a), L. Lundegaard (a), M. McMahon (a), C. Guillaume (a), R. Nelmes (a), M. Mezouar (b), Structural Diversity of Sodium, Science 320, 1054 (2008).
(a) Centre for Science at Extreme Conditions and School of Physics, University of Edinburgh (UK)
(b) ESRF

References

[1] H. Mao et al., J. of Physics-Condensed Matt. 18, S963 (2006).
[2] E. Gregoryanz et al., Phys. Rev. Lett. 94, 185502 (2005).
[3] E. Brovman, Yu. Kagan, A. Kholas, Soviet Physics JETP 34, 1300 (1972); Soviet Physics JETP 35, 783 (1972).