At first glance, lithium should be a simple atomic system. It is the lightest solid element and with just three electrons, it should exhibit simple, crystalline structures. However, an international team of scientists has shown that under high pressure, lithium “prefers” the liquid state, and that it turns out to be the elemental metal with by far the lowest melting point. At high pressure, lithium also undergoes a series of phase changes into surprisingly complex structures. The results of these experiments performed at the ESRF and the APS are published this week in Nature Physics.
At ambient conditions, lithium forms rhombohedral crystals which at higher temperatures and pressures transform to face-centered cubic and then body-centered cubic crystals – all three among the simplest known crystal structures. However, what happens at very high pressures? In the past few years, intriguing deviations from simple metallic behaviour were observed, for example a metal to semiconductor-transition and even superconductivity at 17 K.
Nevertheless, the overall picture of the lithium phase diagram remained patchy, motivating a systematic study by an international team of researchers led by Eugene Gregoryanz from the University of Edinburgh and Michael Hanfland from the ESRF who have mapped the lithium phase diagram at high pressures up to 1.3 Mbar, and over a wide temperature range between 77 and 300 K.
Whereas the melting point of a material usually rises under pressure, and even the lightest gaseous elements, hydrogen and helium, melt at 1000 K and 0.5 Mbar, lithium remains liquid at this pressure down to temperatures as low as 190 K. This is by far the lowest melting temperature observed for any material at this pressure.
Phase diagram of lithium (red) and sodium (blue). The lithium phase diagram indicates the various solid states and also the liquid state at 50 GPa (0.5 Mbar) and temperatures below 200K.
Credit O. Degtyareva
There are more surprises: above 0.6 Mbar, lithium adopts three novel, complex crystal structures not previously observed in any element with 40, 88 and 24 atoms per unit cell. The most complex ones with 40 and 88 atoms were even never predicted theoretically. The scientists suggest that the overall appearance of the lithium phase diagram and particularly the anomalously low melting temperatures are due to quantum effects starting to play the dominant role at high compressions. They also speculate that a ground metallic liquid state, which has been predicted but never observed for hydrogen and which should exhibit highly unusual properties, might be constructed on the basis of the lithium-rich compounds.
The experiments were carried out using powder and single-crystal diffraction at the ESRF High-Pressure Beamline ID09A, with additional single-crystal data collected at the Advanced Photon Source (APS) near Chicago, where also some cryotechniques were refined. The team had indeed to address several experimental challenges: handling a diamond anvil cell in a cryostat, ensuring a wide opening angle to allow for single-crystal diffraction, and last but not least coping with the high reactivity of lithium in particular when in the liquid phase.
Reference: Christophe L. Guillaume et al., Cold melting and solid structures of dense lithium, Nature Physics 9 January 2011 DOI:10.1038/NPHYS1864
Credit: the above photo of pieces of Lithium metal from the Dennis "S.K." collection is published under the GNU Free Documentation License.