Crystallographic Anisotropy in Compression of Uranium Metal to 100 GPa
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Crystallographic Anisotropy in Compression of Uranium Metal to 100 GPa Yogesh K. Vohra1, Kevin M. Hope1, J. Reed Patterson, and Jagannadham Akella L-201, Lawrence Livermore National Laboratory (LLNL), Livermore, CA 94550, U.S.A. 1 Department of Physics, University of Alabama at Birmingham (UAB) Birmingham, AL 35294-1170. ABSTRACT New high-pressure x-ray diffraction data on uranium metal (99.9 %) in a diamond anvil cell is presented to 100 GPa (Volume compression V/Vo = 0.700) at room temperature using a variety of pressure markers like ruby, copper, and platinum. The diffraction patterns are carefully indexed allowing for reversal of peak positions based on anisotropic compression. We report anisotropic compression of the orthorhombic unit cell with the axial ratio b/a increasing initially to 40 GPa followed by a rapid decrease at higher pressure. On the other hand, axial ratio c/a shows a rapid increase with increasing pressure followed by saturation at megabar pressures. The most recent full potential electronic structure calculations reproduce the increasing tend of axial ratio c/a to 100 GPa but do not explain the variation in the b/a ratio. Our detailed analysis of all available experimental data also indicates that the observed anisotropic effects are intrinsic to Uranium and are independent of the pressure medium used in the high-pressure experiments.
INTRODUCTION The light actinide metals (Th-Pu) have always occupied unique placement in the periodic table because of their itinerant 5-f electrons and complex crystallographic phases at high pressures and high temperatures.1 The contributions of 5-f electrons to bonding gives rise to low symmetry crystal structures and a narrow 5f-band pinned to the Fermi-level. The high-pressure studies on 5-f metals have always played a key role in modifying their electronic structure due to band broadening and electron transfer effects among various electronic sub-bands under high pressures. In particular, uranium metal has been a subject of intense theoretical and experimental investigations2-5 and published data to 100 GPa and 4500 K has established the existence of orthorhombic (α-phase), body centered tetragonal (β-phase), and body centered cubic (γ-phase). At room temperature, orthorhombic α-phase is known to be stable to 100 GPa. In a most recent study on uranium metal to 100 GPa with silicon and nitrogen pressure media6, a critical comparison has been made of the equation of state obtained with different pressure mediums. However, changes in axial ratio with pressure and anisotropic compression effects have not been discussed in detail even though changes in diffraction line shapes are readily observable in x-ray studies below 100 GPa. This study is motivated by the anisotropic compression of a, b, and caxis in the orthorhombic uranium and a careful x-ray diffraction analysis has been performed using a variety of pressure markers. We also correlate these anisotropic compression effects in uranium to the changes in electronic structure induced by compression.
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