Cerium's unusual behaviour

Jan 27, 2011 by Anne M Stark
Cerium's unusual behaviour
Photo courtesy of Max Whitby/Red Green & Blue Company

(PhysOrg.com) -- Livermore researchers have found that a crystal of cerium -- the chemical element that can be used for catalysts and fuel additives -- behaves in very unique ways when subjected to high pressures.

When a solid is pressurized, the internal energy at some point produces a change in the atomic arrangement, called a phase transformation. Typically when this happens, the atomic arrangement changes from one geometry to another. However, that doesn't happen in .

When cerium metal is exposed to 75,000 atmospheres of pressure, the of the crystal simply shrinks, is transformed into two . Dan Farber, Kevin Moore, and Chantel Aracne-Ruddle showed that the two crystals are the same structure (one a large cube and the other a small cube) and have the same spatial orientation (the large and small cubes face in the same direction). This means that the two phases have the same , but different volumes. This is entirely unique in phase transformations of .

A phase transformation with two crystals of the same structure, but different volumes is something akin to laying bricks that are the same shape, but where some of the bricks are large while some are small. Making a coherent wall with the two is difficult.

The team (which also included researchers from Université Pierre et Marie Curie, Place Jussieu and CEA, DAM, DIF, all in France) used a diamond anvil cell to create the intense pressures on the cerium crystal. They also applied the X-ray diffraction method to determine the high pressure variation of the cerium structure and volume.

"Our data clearly show that the transformation mechanism can be described on the basis of crystallographic and thermodynamic arguments, showing a fair agreement with an isomorphic scenario and the existence of a critical point," Moore said. "The equation of state of cerium near the critical temperature is determined experimentally and for the first time is shown to be well understood in the framework of the scaling theory of the liquid-gas transition of classical systems. This conclusion represents an important step forward in achieving a reliable and unambiguous picture on the mechanism of phase transformation in cerium, an element archetypical of the localization-delocalization phenomenon encountered in f-electron systems, such as plutonium."

The research is scheduled to appear in a future edition of Physical Review Letters.

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User comments : 11

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kevinrtrs
2 / 5 (6) Jan 27, 2011
We live in a totally, absolutely magical universe. Enjoy it while you can.

It still boggles the mind that atoms on either side of Cerium on the periodic table can behave completely differently under such gigantic pressures. Yet they only differ by the addition or subtraction of one proton and electron: Praseodymium and Lanthanum.
Even in the same column, Thorium and Dubnium are as different as day from night under pressure. Go figure!
bugmenot23
not rated yet Jan 27, 2011
while you can.

Why certainly :)
Question
5 / 5 (2) Jan 27, 2011
We live in a totally, absolutely magical universe. Enjoy it while you can.

It still boggles the mind that atoms on either side of Cerium on the periodic table can behave completely differently under such gigantic pressures. Yet they only differ by the addition or subtraction of one proton and electron: Praseodymium and Lanthanum.
Even in the same column, Thorium and Dubnium are as different as day from night under pressure. Go figure!

Atoms have shape. Add or subtract a single electron changes the outside surface charge shape.

axemaster
not rated yet Jan 27, 2011
Atoms have shape. Add or subtract a single electron changes the outside surface charge shape.

Pretty sure that's not the case, but then I'm not an expert on the matter. Rather, I'd expect it to change the available energy levels and the mean radius of the atom...

Actually I take that back... I just remembered that the nuclei of very large atoms are often elongated because of the nucleus energy levels. I'm not sure if that significantly changes the shape of the electron orbitals though.
malapropism
not rated yet Jan 27, 2011
Pedantic pickiness for the day:
behaves in very unique ways

Come on Physorg writers & editors, I don't normally comment on the standard of writing as it's generally ok but I think this one is really taking grammar a bit too far (admittedly because it's in the teaser as well as the article text). Something is either unique or it is not, it cannot be "very unique" nor any other qualified version of uniqueness.
YawningDog
not rated yet Jan 27, 2011
Physorg doesn't have writers & editors. They just cut and paste what they find elsewhere.
YawningDog
not rated yet Jan 27, 2011
What caught my eye and clicked me here was seeing something about "fuel additive".

I thought, Oh God! We got rid of lead in the fuel and now along comes cerium. It won't stop until we're all three eyed monsters.
rgwalther
not rated yet Jan 27, 2011
What caught my eye and clicked me here was seeing something about "fuel additive".

I thought, Oh God! We got rid of lead in the fuel and now along comes cerium. It won't stop until we're all three eyed monsters.


Weird! You only have three eyes?
Physphan
not rated yet Jan 28, 2011
...and the eyes have it.

Thought-provoking article.
Macksb
1 / 5 (1) Jan 29, 2011
This is a long shot, but I'll mention it anyway. I wonder if this is an example of a chimera state of coupled oscillators. See 2008 PRL article by Abrams, Mirollo, Strogatz, Wiley: "Solvable Model for Chimera States of Coupled Oscillators." A network of identical oscillators can spontaneously split into two units: a synchronized (smaller cube?) and a desynchronized (larger cube?)population.
Moebius
not rated yet Jan 29, 2011
When cerium metal is exposed to 75,000 atmospheres of pressure, the atomic arrangement of the crystal simply shrinks, is transformed into two crystals.


Should this sentence actually say "When cerium metal is exposed to 75,000 atmospheres of pressure, the atomic arrangement of the crystal doesn't simply shrink, it is transformed into two crystals."?