Article

Theoretical zero-temperature phase diagram for neptunium metal.

Physical review. B, Condensed matter (Impact Factor: 3.66). 08/1995; 52(3):1631-1639. DOI: 10.1103/PhysRevB.52.1631
Source: PubMed

ABSTRACT Electronic structure calculations, based on the density-functional theory with the generalized gradient approximation to the exchange and correlation energy, are used to study the crystallographic properties of neptunium metal under compression. Calculated ground-state properties, such as crystal structure and atomic volume, are found to be in excellent agreement with experiment. The calculated bulk modulus and the first pressure derivative of the bulk modulus are also in accordance with experiment. Our theory predicts that neptunium at low temperature undergoes two crystallographic phase transitions upon compression. First a transition alpha-Np-->beta-Np at 0.14 Mbar and second a beta-Np-->bcc phase transition at 0.57 Mbar. These transitions are accompanied by small volume collapses of the order of 2-3 %. The high-pressure phase (bcc) is also investigated theoretically up to 10 Mbar. A canonical theory for the f-electron contribution to the hcp, fcc, and bcc structures is presented.

0 Bookmarks
 · 
161 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Diamond-anvil cell experiments and first-principles theory have been used to investigate the structural stability of uranium up to 1 Mbar in pressure. Experiments and theory agree; there is no phase transition in uranium below 1 Mbar. Previous speculations about a crystallographic phase transition in uranium below this pressure are thus shown to be incorrect. In this regard, uranium is exceptional in the series of light actinides, where pressure-induced phase transitions typically occur at pressure below 1 Mbar. The ground-state crystal structure of uranium is orthorhombic with three structural parameters: the axial ratios b/a and c/a, and an internal parameter y measuring the displacement, along the b-axis, of alternate planes. The experimental and theoretical results reported here indicate that one of these parameters, c/a, is substantially more sensitive to pressure than the other two, changing by as much as 5%, while b/a and y are constant within 1% within the pressure range studied. This flexibility in the 0953-8984/9/39/003/img7 structure facilitates this structure over a wide pressure range. Theory suggests that electrostatic contributions to the total energy drive the variation in the c/a ratio as a function of pressure, and a simple model is utilized to show this.
    Journal of Physics Condensed Matter 09/1997; 9(39):L549-L555. · 2.22 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Density-functional theory, previously used to describe phase equilibria in the γ-U-Mo alloys [A. Landa, P. Söderlind, P.E.A. Turchi, J. Nucl. Mater. 414 (2011) 132], is extended to study ground-state properties of the bcc-based (γ) X-Mo (X = Np, Pu, and Am) solid solutions. We discuss how the heat of formation correlates with the charge transfer between the alloy components, and how magnetism influences the deviation from Vegard's law for the equilibrium atomic volume.
    Journal of Nuclear Materials 03/2013; 434(1):31-37. · 2.02 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Norm conserving pseudopotentials have been generated for the light actinides (Th-Np) and the plane waves + pseudopotential formalism has been used to study their crystal structures at zero temperature as a function of pressure. The often complex alpha phases of these elements have been fully relaxed, and we have used a thorough treatment of spin-orbit coupling. The zero-pressure zero-temperature equilibrium volumes and bulk moduli are consistent with previous all-electron full-potential calculations, and, up to uranium, in excellent agreement with experiment. This is also the case for cell parameters and pressure-induced phase transitions. It is likely that, from neptunium on, a more careful treatment of electronic correlations and/or relativistic effects is necessary to reproduce the experimental data with the same precision.
    Physical Review B 12/2002; · 3.66 Impact Factor

Full-text

Download
5 Downloads
Available from
May 24, 2014