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Achieving the pressure at Earth's core-mantle boundary is now plausible. The pressure and temperature conditions that humans commonly experience on Earth are but a small subset of the conditions to which most materials in the universe are subjected. These conditions include very low pressures in outer space ranging up to very high pressures at the centers of neutron stars, as well as temperatures ranging from nearly absolute zero to many thousands of degrees kelvin.
Throughout the universe pressure is expected to span more than 60 orders of magnitude. At very high pressures the electronic structure of atoms is altered as positions of electrons orbiting nuclei are changed, leading to unexpected chemical interactions. Scientists' current understanding of the electronic structure of atoms in elements on Earth forms the basis of modern chemistry's set of rules. Under extreme pressures and temperatures, the electronic structure of chemical bonds would likely change as electrons are squeezed between atoms, leading to different "rules" of chemical interaction. Knowing the new rules might allow researchers to more easily synthesize new materials and engineer materials to meet ever-increasing demands. Scientists have created extreme conditions in the laboratory for many decades. A particularly difficult task has been to probe the properties of materials subjected to extreme conditions. The Spallation Neutrons and Pressure (SNAP) Beamline instrument at the Spallation Neutron Source, which first opened its neutron beam shutter on Jan. 24, 2008, has the potential of exerting pressures near those at the boundary between Earth's mantle and iron core (~100 gigapascals) on a wide range of materials and simultaneously performing neutron scattering studies of these materials under extreme conditions. Collecting neutron scattering data under these extreme pressure conditions would be another world record for Oak Ridge National Laboratory. Chris Tulk, a condensed matter physicist who oversaw the construction of the SNAP instrument, is eager to do scientific experiments in which neutrons are used to determine changes in a material's structure after a sample is placed under high pressure over a wide range of temperatures. He says that SNAP's suite of pressure-generating devices can easily cause changes in a material's molecular bonding, crystallographic structure and interactions of atoms. The increased neutron flux of SNS, combined with SNAP's large-volume, gem anvil and gas pressure cells, will enable researchers to conduct neutron diffraction experiments over a large range of pressures and temperatures never before available in the United States. "At high pressure, a sample's crystal structure, or the spatial distribution and bonding of the elements, could transform from one set of symmetry operations to another," says Tulk. "Simply to accommodate the decrease in volume resulting from the increase in pressure, the sample thus transforms from one crystallographic structure to another. "Changes in electrons' ranges of energies can cause a material to go from electrically insulating to conducting to even superconducting. Scientists still struggle to understand these changes." Using neutron scattering, Tulk expects to observe atomic-level changes as hydrogen gas is compressed enough to becomea crystal, as water is pressurized along with natural gases to simulate methane hydrates deep in the ocean, and as protein samples are subjected to the same temperatures and pressures as extremophile microbes found in thermal vents deep in the ocean. Magnetic properties of molten iron under pressure might help researchers better understand Earth's core and magnetic field. "SNAP was conceived and built to be a user facility," Tulk says. "We are eager to engage with other research groups at ORNL and throughout the world to enhance their scientific programs."—Carolyn Krause
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