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Actinide Element Curium (Cm) Has Strange Crystal Structure

How the crystal structure of some metals becomes stable through magnetism

By Lucian Dorneanu, Science Editor

7th of June 2007, 09:14 GMT

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Curium (Cm) is a strange element, a heavy actinide that behaves strangely under extreme pressure. Now, a new study provides a better understanding of how the crystal structure of some metals becomes stable through magnetism.

It's a synthetic radioactive metallic transuranic element of the actinide series, produced by bombarding plutonium with alpha particles (helium ions) and was named after Marie Curie and her husband Pierre.

The element has a complex crystal structure, produced by magnetic stabilization. This process is rare in metals and large enough to influence the crystal structure in some metals, such as manganese, iron and cobalt.

A new study by a team of scientists from Lawrence Livermore and Oak Ridge national laboratories and Daresbury Laboratory in the United Kingdom now showed that magnetically stabilized crystal structures also include the heavy actinide element, curium (Cm).

The diamond-anvil study subjected curium to pressures of up to one million atmospheres and that caused the metal to undergo transformations between five different crystal phases. This was done by probing the electronic and magnetic structure of the element with electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM), electron atomic calculations and density functional theory (DFT).

"Our results for curium go a long way in teaching us a general understanding of how this mechanism occurs," said Kevin Moore, the LLNL, lead author of the research paper.

The principle used in the applications was the Hund's rule of maximum spin multiplicity, which states that a greater total spin state usually makes the resulting atom more stable, most commonly exhibited in a lower energy state, because it forces the unpaired electrons to reside in different spatial orbitals.

This rule deals again with reducing the repulsion between electrons. It can be understood from the classical picture that if all electrons are orbiting in the same direction (higher orbital angular momentum) they meet less often than if some of them orbit in opposite directions. In that last case the repulsive force increases, separating the electrons. This adds potential energy to them, so their energy level is higher.

"This gives us great insight into the valence state and electron coupling mechanisms of 5f electrons in plutonium and americium, two metals that are significant to nuclear reactors," Moore said. "Our data will help us refine our theoretically predictive codes for these metals to give us a better understanding of the physical properties of the metals and how they will behave under extreme conditions."

TAGS:

element | plutonium | electron | crystal

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