Cerium enters a rare and spectacular phase


As a solid is subjected to higher pressures, its atomic arrangement changes. For most elements, the transition is accompanied by a decrease in volume, which means that a single crystal crushes into a polycrystal or a powder. But cerium, the most abundant rare-earth element with applications ranging from catalysts to fluorescent lamps, does not conform to this picture. At a temperature of 300 K and pressure of 0.75  GPa, the volume of cerium decreases without any accompanying change in its structure.

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ESRF users have now shed light on the mechanism behind this counter-intuitive transition. Frédéric Decremps of the Université Pierre et Marie Curie (IMPMC) and co-workers from the IMPMC, Lawrence Livermore National Laboratory and the CEA took the world’s only existing single crystal of cerium to the ESRF’s ID09 beamline, where X-ray diffraction experiments revealed its non-equilibrium phase diagram. 

When the sample was subjected to high pressures in a diamond anvil cell, it underwent a transition that has strong analogies with the liquid-gas transition of classical systems – a phenomenon that has never been observed in any solid. The initial phase of cerium transforms to a new phase with about 15% less volume. Yet the team observed that the single-crystal quality remains and, more surprisingly, two single crystals with different volumes coexist. The researchers also determined the high-pressure variation of volume in a single crystal of cerium along different isotherms. “This a major first in crystallography,” Decremps told ESRFnews.

In 2009, Decremps and co-workers showed that the variation of volume during the phase transition is provoked by a mechanical anomaly: the more solid cerium is compressed, the more it becomes compressible – behaviour that had never been observed in a pure element. The new work sheds light on transformations generated by the delocalisation of f-electrons, which could be the cause of the exceptional mechanical properties of cerium-based metallic glasses. These have promising technological applications because they deform plastically at almost ambient temperatures.

“A bright and small X-ray beam was vital to obtain high-quality data on cerium’s equation of state at high pressure and high temperature,” says Decremps. “This experiment is feasible only at the ESRF on beamline ID09, which is partly devoted to high-pressure high-temperature experiments and has enough flux to collect excellent data on a small area of the sample.” 


F Decremps et al., Phys. Rev. Lett. 106, 65701 (2011).


This article appeared in ESRFnews, June 2011. 

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Top image: Rare-earth element: cerium (Credit: T Gray/periodictable.com).