Theoretical study of protactinium at high pressure
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0893-JJ06-06.1
Theoretical study of protactinium at high pressure Börje Johansson1,2,3, Sa Li2,4, Eyvaz Isaev5 and Rajeev Ahuja1,2 1
Condensed Matter Theory Group, Department of Physics,
Uppsala University, BOX 530, S-751 21, Uppsala, Sweden 2
Applied Materials Physics, Department of Materials Science and Engineering,
Royal Institute of Technology, Brinellvägen 23, SE-100 44, Stockholm, Sweden 3
AB Sandvik Materials Technology, SE-811 81 Sandviken, Sweden 4
Department of Physics, Virginia Commonwealth University, Richmond, VA 23284, USA
5
Theoretical Physics Department, Moscow State Institute of Steel and Alloys (Technological University), 4 Leninskii prospect, Moscow 119 849, Russia Abstract
We have studied the crystal structure of Pa metal under high pressure by means of firstprinciples calculations based on the density functional theory (DFT) using the generalized gradient approximation (GGA). The body centered tetragonal (bct) to orthorhombic (α-U) phase transition was calculated to take place at 29 GPa and with a volume change of 1.3%. The calculated c/a for the bct phase reaches the ideal c/a value (0.816) at around 50 GPa. A bulk modulus of 113 GPa was derived from a Murnaghan equation of state (EOS) fitting procedure. Our results are in general good agreement with recent experiment performed by Haire et al. [Phys. Rev. B 67, 134101 (2003)]. We have also calculated phonon spectra for fcc, bct and bcc Pa. The bcc spectrum gives imaginary frequencies showing the low temperature instability of this crystallographic phase for Pa.
0893-JJ06-06.2
Introduction The variation of the atomic volume for the early actinide metals originates from the gradually increasing occupation of the 5f orbitals as one proceeds to higher atomic numbers. Thorium is the first element in the actinide series with empty 5f orbitals in the free atom - it behaves as a transition metal atom with a 6d27s2 electron configuration –while in the condensed phase its fcc structure actually reflects a small but still substantial occupation of the 5f orbitals. Protactinium, being the element following thorium in the series, is of particular significance since it is the first element in the actinide series where a significant portion of the five valence band electrons has the signature of itinerant 5f electrons. This is so already at atmospheric pressure and therefore Pa occupies an important position in the actinide series. With itinerant 5f electrons participating in the metallic bonding at ambient conditions, the four elements from Pa to Pu exhibit lower symmetry crystal structures, smaller equilibrium volumes and higher bulk modulus than the other actinide elements.1 Arriving at Am, the 5f electrons have become localized2, 3 and do not participate in the bonding. As a consequence the atomic volume of americium is almost 50% larger than for the preceding element plutonium4. For the elements after Am, namely, Cm5, Bk and Cf, their atomic volumes are very similar to that of Am. As early as in the 1960’s, superconductivity in protactinium
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