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Spin transition in Earth’s lower mantle
Livermore researchers working in collaboration with universities and other scientific institutions have located the spin-transition zone of iron in Earth’s lower mantle. Scientists determined the location by studying the electronic spin state of iron in ferropericlase (iron magnesium oxide) at high temperatures and pressures similar to those in the lower mantle. Their research appeared in the September 21, 2007, issue of Science.

The lower mantle makes up more than half Earth’s volume. The spin-transition zone is the region where the electronic spin of iron in mantle minerals changes from the high- to low-spin state. In the Livermore collaboration, scientists studied the electronic spin state and crystal structure of iron in ferropericlase under lower mantle conditions using x-ray emission spectroscopy and x-ray diffraction with a laser-heated diamond anvil cell. Through their research, they identified the mix of iron’s high- and low-spin states that is likely to occur in the spin-transition zone. The transition of iron in ferropericlase changes the material’s density, elasticity, electrical conductivity, and other transport properties.

Ferropericlase is the second most abundant mineral in the lower mantle, and its physical properties are important for understanding Earth’s structure and composition. By observing the spin state, scientists can better understand Earth’s structure, composition, and dynamics, which affect geologic activities on the planet’s surface. In addition, the techniques developed through this research will allow researchers to study how lanthanoid and actinoid compounds react under extreme pressures.

Contact: Jung-Fu Lin (925) 424-4157 (lin24@llnl.gov).

Carbon nanotube interactions on the atomic scale
Collaborators from Lawrence Livermore and several other institutions have demonstrated how carbon nanotubes interact with chemical functional groups on the atomic scale. The researchers used chemical force microscopy, a nanoscale technique for determining interaction forces, to measure for the first time a specific interaction between a single functional group and a nanotube.

Functional groups are the smallest specific collection of atoms within a molecule that control the molecule’s characteristic chemical reactions. The study results, which appeared in the November 2007 edition of Nature Nanotechnology, indicate that interaction strength does not follow conventional trends of water repulsion or increased polarity, but rather depends on the intricate electronic interactions between the nanotube and the functional group. Because nanotubes are so small, researchers previously have relied on modeling, indirect measurements, and microscale tests to measure the adhesion force of an individual molecule at the carbon nanotube surface. The Livermore team achieved a more exact measurement by reducing the size of the probe–nanotube contact area.

The team then collaborated with computational chemists to simulate the functional group–nanotube interactions. Calculated interaction forces provided an exact match to experimental results. According to Livermore researcher Aleksandr Noy, “In the past, there was a gap between what we could measure in an experiment and what the computational methods could do. It is exciting to be able to bridge that gap.” The ability to measure interactions on a single functional group level could allow for more accurate and precise designs of new nanocomposite materials, nanosensors, and molecular assemblies.

Contact: Aleksandr Noy (925) 424-6203 (noy1@llnl.gov).

 


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