Pressure turns simplicity into complexity

That coal can morph into the hardest and most highly prized structure known, diamond, is a dramatic example of high-pressure physics in action. When an element is subjected to very high pressures, its interatomic distances are reduced and its electronic structure shifts markedly with respect to the same system at ambient conditions. Pressure is therefore a powerful tool to probe the relationship between the structure of materials and their properties.

Advances in synchrotron instrumentation have brought about a renaissance in mapping out the phase diagrams of the simplest systems, such as hydrogen, lithium and sodium. The ESRF provides X-ray diffraction and spectroscopy studies of samples compressed to pressures more than two million times greater than atmospheric pressure using diamond anvil cells, and subjected to temperatures ranging from 5 K, in a cryostat, to 5000 K using lasers.

“There has been great progress during the past 20 years in our understanding the fundamental physics of condensed matter under extreme conditions, thanks in part to third-generation synchrotrons,” says Paul Loubeyre of the CEA in Bruyères-le-Châtel, France. Structural, magnetic, dynamic and electronic properties can now be measured with similar accuracy and resolution as if the material were at ambient pressure, he explains, and many old paradigms ruling the high-pressure behaviour of materials have been proven to be not so simple: for instance, the evolution to close packing structures, the breaking of molecular bonds or the systematic evolution to a metallic state. “The reality is indeed richer and more complex than anticipated,” he says.

Rich testing ground

The simplicity of hydrogen and the alkali metals makes them a rich testing ground for fundamental physics, since nowadays such systems can be modelled in sufficient detail to allow close comparison between theory and experiment. “The holy grail is to produce metallic hydrogen, and to convincingly prove its existence,” explains the ESRF’s Michael Krisch.

Sodium’s phases have been the subject of intense scrutiny at the ESRF, and raised several surprises. In 2008, for instance, a study by Eugene Gregoryanz from Edinburgh University, UK, and colleagues unveiled “extraordinary” liquid and solid states of sodium at pressures above 100 GPa that involved seven different crystalline phases (Science 320 1054), while an earlier result by the same group showed a large, pressure-induced drop in sodium’s melting point at high pressures (Phys. Rev. Lett. 94 185502). Similar oddities have been found in the lighter alkali metal lithium. Above pressures of 60 GPa, lithium adopts novel crystal structures with up to 88 atoms per unit cell. Both sodium and lithium lose their metallic state under pressure and become semiconductors (Nature Physics 7 211).

Solid oxygen, which is a slightly more complex diatomic molecule, has been turned into a metal at pressures above 96 GPa and, uniquely for a molecular state, becomes a superconductor at very low temperatures. “The structure is vital to know what’s going on, and a big surprise has been observed,” says Loubeyre: in the solid phase, O₂ molecules associate with pressure to form O₈ entities that ultimately connect to form the metallic state (Phys. Rev. Lett. 102 255503). The other archetypal diatomic molecule, N₂, has a completely different evolution under pressure, he adds: the triple bond is broken and a polymeric phase of single-bonded nitrogen has been observed (Nature Materials 3 558). “Interestingly, recent calculations predict that at pressure in excess of 300 GPa nitrogen will form a new structure with N₁₀ that has never been observed.” 

The new periodic table

Essentially, says Loubeyre, researchers have a new periodic table at pressures of 100 GPa, and therefore a new landscape of condensed matter physics to understand. Most of the work so far has been performed on the novel structures adopted by elemental solids at high pressure, but many more surprises remain to be discovered by understanding the new chemical rules of this table so as to synthesise new compounds. One of the key questions to be addressed is whether the properties of elements and compounds apparent at high pressure can be recovered in ambient conditions. “We can call it the emergence of complexity of high pressure,” Loubeyre explains. “For example, van der Waals compounds exist only under pressure and are now observed ubiquitously, while many other interesting new compounds are being predicted, such as LiH₆.”

With the far corners of the phase diagrams of the simplest elemental solids becoming well explored, researchers are turning to the more complex rare-earth metals: the lanthanides. Cerium surprised ESRF users Frédéric Decremps of the Université Pierre et Marie Curie and co-workers last year, when they subjected a unique single crystal of cerium to a pressure of 0.75 GPa. Unlike most solids under pressure, which undergo a decrease in volume due to changes in their atomic structure, cerium’s volume decreased without any accompanying change in its structure. In fact, the team claims that despite the single-crystal nature of the sample the transition is more like the liquid-gas transition of classical systems (Phys. Rev. Lett. 106 65701).

This year, an experiment conducted jointly between the ESRF and users from Edinburgh University put heavier europium, which is divalent rather than trivalent, under pressure (Phys. Rev. Lett. 109 095503). The team uncovered a new phase in the lanthanides – an “incommensurately modulated crystal structure” – in which the atoms are displaced by a small amount from their expected crystal location. The study, says the team, motivates further density-function-theory calculations to uncover the mechanism behind europium’s strange high-pressure behaviour, which includes becoming superconductor with a transition temperature of 1.8 K.

The high-pressure periodic table has also gained its first actinide, uranium, which has a complex ground state and becomes a superconductor at pressures of 1.5 GPa. These systems are still a long way away from “real life” materials, but by building up an understanding of the simplest systems researchers hope to shed light on more complex ones. “These things were not possible a few years ago,” says the ESRF’s Michael Hanfland, “The past 5–10 years has seen the development of ab initio prediction of structures thanks to new efficient algorithms and ever-increasing computing power.”

Matthew Chalmers

Planetary interiors laid bare
Inside the distant, cold planets Neptune and Uranus, where pressures reach 300 GPa and temperatures top 5000 K, strange phases of water, ammonia and methane ice are predicted to exist. At moderate pressures and temperatures, ammonia ice (NH₃) is a molecular crystal similar to water ice. But in 1999, theorists calculated that ammonia enters a new phase – superionic ammonia ice – under the extreme conditions of planetary interiors. Superionicity is an exotic state of matter that behaves simultaneously as a crystal and as a liquid.
This year, ESRF users from the Université Pierre et Marie Curie in Paris observed the new  ammonia state – called the alpha phase – for the first time (Phys. Rev. Lett. 108 165702). It occurred at pressures above 60 GPa and temperatures above 750 K, which is slightly warmer than the molecular dynamics simulations suggest. X-ray diffraction showed that ammonia’s nitrogen sub-lattice is the same as in the low temperature solid, but that the hydrogen atoms are much less ordered. Superionic ice that exists inside Neptune and Uranus is suspected to be the origin of their magnetic fields.
Experiments at the ESRF are also tackling our own planet’s interior, namely its hot dense iron core. Last year the ID24 beam line was inaugurated, where powerful pulsed lasers will reproduce these extreme environments for a few microseconds at a time. Such setups allow the liquid phase to be explored at pressure, in particular to find out how liquids evolve from a dense molecular state to a plasma. “Warm dense matter plays an important role in stellar interiors and fusion research,” says the ESRF’s Michael Hanfland. “But it’s also a new state of matter that can’t be simulated very easily.”

Towards metallic hydrogen
Even familiar systems can shock physicists when put under pressure. Take the evolution of elemental melting curves and the structural changes in the dense fluid phase. The melting point of iron, for instance, shows discrepancies between theory and measurement at high pressures, which is important for studies of the Earth’s interior. Recently, the use of X-ray synchrotron diffraction has been shown to be crucial in obtaining a correct determination of the melting curve, as illustrated in the case of tantalum (Phys. Rev. Lett. 104 255701). Longer term, the aim is to extend the temperature domain in order to bridge the gap between condensed matter physics and plasma physics – a domain called warm dense matter. That will probably require adapting dynamic compression techniques to synchrotrons.
But it is the simplest system of all – hydrogen – that has confounded physicists the most. Hydrogen’s metallic phase, predicted since the 1930s, has still not been unambiguously observed. Furthermore, the structural changes of solid hydrogen along its route to metal hydrogen, as measured by single crystal X-ray diffraction at the ESRF, are challenging because they are driven by nuclear quantum effects (e.g. Nature 435 1206). Ultimately, dense hydrogen should create a new form of matter called a quantum liquid that may be both a superconductor and a superfluid near absolute zero, which would be extremely hard to characterise. This year physicists at the Max Planck Institute for Chemistry in Mainz, Germany, claimed to have observed the metallic transition in a diamond anvil cell at 300 K (Nature 486 174), but the jury is still out.
“The precise pressure at which it happens is not known, but metallic hydrogen should exist,” says Patrick Bruno, head of the ESRF’s theory group. “The discovery would probably be worth a Nobel prize.”

This article originally appeared in ESRFnews, December 2012. 

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Top image: Sodium, which is a soft metal under ambient conditions, reveals a different side at high pressure. (Image credit: S.K.” collection published under the GNU Free Documentation License).