Investigations of Actinide Metals and Compounds under Pressure Provide Important Insights into Bonding and Chemistry
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Investigations of Actinide Metals and Compounds under Pressure Provide Important Insights into Bonding and Chemistry R. G. Haire1, S. Heathman2, T. Le Bihan3, A. Lindbaum4 and M.Iridi2 1
Oak Ridge National Laboratory, CSD, P. O. Box 2008, MS-6375, Oak Ridge, TN 37831 European Commission, JRC, Institute for Transuranium Elements, Postfach 234, D-76125 Karlsruhe, Germany 3 European Synchrotron Radiation Facility, B.P.220, F-38043, Grenoble Cedex, France 4 Vienna University of Technology, Institute for Solid State Physics, Wieder Haupstrasse 810/138, Wien, Autstra 2
ABSTRACT One effect of pressure on elements and compounds is to decease their interatomic distances, which can bring about dramatic perturbations in their electronic nature and bonding, which can be reflected in changes in physical and/or chemical properties. One important issue in the actinide series of elements is the effect of pressure on the 5f-electrons. We have probed changes in electronic behavior with pressure by monitoring structure by X-ray diffraction, and have studied several actinide metals and compounds from thorium through einsteinium. These studies have employed angle dispersive diffraction using synchrotron radiation, and energy dispersive techniques via conventional X-ray sources. The 5f-electrons of actinide metals and their alloys are often affected significantly by pressure, while with compounds, the structural changes are often not linked to the involvement of 5 f-electron. We shall present some of our more recent findings from studies of selected actinide metals, alloys and compounds under pressure. A discussion of the results in terms of the changes in electronic configurations and bonding with regard to the element’s position in the series is also addressed. INTRODUCTION Pressure, like temperature, is an important variable in the chemistry, physics and materials science of materials. It is of special interest in geochemistry, where very significant changes in the science of materials may be encountered with pressures in the depths of the earth. With the actinide elements, the 5f electron series of elements can be affected by pressure. Even at room temperature and atmospheric pressure these electrons are found to play a changing role across the series. At atmospheric pressure the Pa, U, Np and Pu elements have itinerant 5f electrons (electrons involved in the metallic bonding) and these metals exhibit unusual properties and display low-symmetry structures. Plutonium may be the most complex metal in the Periodic Table, due in part to the existence of multiple electronic levels of similar energies that involve its 5f-electrons. In contrast, the transplutonium metals at atmospheric pressure have localized 5f-electrons, have symmetrical structures and for many their bonding consists of three non-f-electron conduction electrons (i.e., “trivalent metals”) and in this regard mimic most of the lanthanide metals. This difference in bonding encountered across the actinide series affects strongly the science and physical behavior of
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