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Phase stability in heavy f-electron metals from first-principles theory

Description: The structural phase stability of heavy f-electron metals is studied by means of density-functional theory (DFT). These include temperature-induced transitions in plutonium metal as well as pressure-induced transitions in the trans-plutonium metals Am, Cm, Bk, and Cf. The early actinides (Th-Np) display phases that could be rather well understood from the competition of a crystal-symmetry breaking mechanism (Peierls distortion) of the 5f states and electrostatic forces, while for the trans-plutonium metals (Am-Cf) the ground-state structures are governed by 6d bonding. We show in this paper that new physics is needed to understand the phases of the actinides in the volume range of about 15-30 {angstrom}{sup 3}. At these volumes one would expect, from theoretical arguments made in the past, to encounter highly complex crystal phases due to a Peierls distortion. Here we argue that the symmetry reduction associated with spin polarization can make higher symmetry phases competitive. Taking this into account, DFT is shown to describe the well-known phase diagram of plutonium and also the recently discovered complex and intriguing high-pressure phase diagrams of Am and Cm. The theory is further applied to investigate the behaviors of Bk and Cf under compression.
Date: November 17, 2005
Creator: Soderlind, P
Partner: UNT Libraries Government Documents Department

Cancellation of spin and orbital magnetic moments in (delta)-Pu: theory

Description: Density functional theory (DFT), in conjunction with the fixed-spin-moment (FSM) method, spin-orbit coupling (SO), and orbital polarization (OP), is shown to retain key features of the conventional DFT treatment of {delta}-Pu while at the same time not producing the substantial net magnetic moments commonly predicted by this theory. It is shown that when a small adjustment of the spin moment (less than 20%) is allowed, a complete spin- and orbital-moment cancellation occurs which results in a zero net magnetic moment in {delta}-Pu. This minor modification, accomplished by the FSM method, is shown to have a very small effect on the calculated total energy as well as the electron density-of-states (DOS). The photoemission spectra (PES), obtained from the DOS of the present model, compares equal or better to measured spectra, than that of two other recent non-magnetic models for {delta}-Pu.
Date: June 23, 2006
Creator: Soderlind, P
Partner: UNT Libraries Government Documents Department

Density-functional electronic structure of PuCoGa5

Description: Density-functional electronic-structure calculations for PuCoGa{sub 5} are performed to address the possibility of magnetic interactions in this high-temperature superconductor. Within an itinerant 5f-electron picture, cohesion and crystallographic parameters compares favorably with experiment, whereas only when spin and orbital interactions are accounted for the calculated electronic density of states agrees with photoemission spectra. This fact suggests that spin and orbital correlations are important for a correct description of the PuCoGa{sub 5} electronic structure and may play a role in an unconventional mechanism for superconductivity.
Date: June 3, 2004
Creator: Soderlind, P
Partner: UNT Libraries Government Documents Department

An Alternative Model for Electron Correlation in Pu

Description: Using a density functional theory based approach that treats the 5f electrons relativistically, a Pu electronic structure with zero net magnetic moment is obtained, where the 5f orbital and 5f spin moments cancel each other. By combining the spin and orbital specific densities of states with state, spin and polarization specific transition moments, it is possible to reconstruct the experimentally observed photoemission spectra from Pu. Extrapolating to a spin-resolving Fano configuration, it is shown how this would resolve the extant controversy over Pu electronic structure.
Date: October 23, 2007
Creator: Yu, S; Tobin, J & Soderlind, P
Partner: UNT Libraries Government Documents Department

Melting of Xenon to 80 GPa, p-d hybridization, and an ISRO liquid

Description: Measurements made in a laser heated diamond-anvil cell are reported that extend the melting curve of Xe to 80 GPa and 3350 K. The steep lowering of the melting slope (dT/dP) that occurs near 17 GPa and 2750 K results from the hybridization of the p-like valence and d-like conduction states with the formation of clusters in the liquid having Icosahedral Short-Range Order (ISRO).
Date: July 26, 2005
Creator: Ross, M; Boehler, R & Soderlind, P
Partner: UNT Libraries Government Documents Department

FIRST-PRINCIPLES PHASE DIAGRAM OF THE Ce-Th SYSTEM

Description: Actinide physics has seen a remarkable focus the last decade or so due to the combination of improved experimental diamond-anvil-cell techniques and the development of fast computers and more advanced theory. All f-electron systems are expected to have multiphase phase diagrams due to the sensitivity of the f-electron band to external influences such as pressure and temperature. For instance, compression of an f-electron metal generally causes the occupation of f-states to change due to the shift of these bands relative to others. This can in some cases, as in the Ce-Th system, cause the crystal to adopt a lower symmetry structure at elevated pressures. Here we study the phase stabilities of Ce, Th, and the Ce-Th system as a function of compression. Theoretically, both Ce and Th metals are rather well described within the DFT, although a proper treatment of the Ce-Th alloys has not yet been presented. In the present paper we revisit this problem by applying the modern theory of random alloys based on the coherent potential approximation (CPA).
Date: March 13, 2005
Creator: Landa, A & Soderlind, P
Partner: UNT Libraries Government Documents Department

RELATIVISTIC EFFECTS ON THE EQUATION OF STATE OF THE LIGHT ACTINIDES

Description: The effect of the relativistic spin-orbit (SO) interaction on the bonding in the early actinides has been investigated by means of electronic-structure calculations. Specifically, the equation of state (EOS) for the face-centered cubic (fcc) model systems of these metals have been calculated from the first-principles density-functional theory (DFT). Traditionally, the SO interaction in electronic-structure methods is implemented as a perturbation to the Hamiltonian that is solved for basis functions that explicitly do not depend on SO coupling. Here this approximation is shown to compare well with the fully relativistic Dirac treatment. It is further shown that SO coupling has a gradually increasing effect on the EOS as one proceeds through the actinides and the effect is diminished as density increases.
Date: November 4, 2005
Creator: Landa, A & Soderlind, P
Partner: UNT Libraries Government Documents Department

Quantifying the importance of orbital over spin correlations in delta-Pu within density-functional theory

Description: Spin and orbital and electron correlations are known to be important when treating the high-temperature {delta} phase of plutonium within the framework of density-functional theory (DFT). One of the more successful attempts to model {delta}-Pu within this approach has included condensed-matter generalizations of Hund's three rules for atoms, i.e., spin polarization, orbital polarization, and spin-orbit coupling. Here they perform a quantitative analysis of these interactions relative rank for the bonding and electronic structure in {delta}-Pu within the DFT model. The result is somewhat surprising in that spin-orbit coupling and orbital polarization are far more important than spin polarization for a realistic description of {delta}-Pu. They show that these orbital correlations on their own, without any formation of magnetic spin moments, can account for the low atomic density of the {delta} phase with a reasonable equation-of-state. In addition, this unambiguously non-magnetic (NM) treatment produces a one-electron spectra with resonances close to the Fermi level consistent with experimental valence band photoemission spectra.
Date: July 27, 2007
Creator: Soderlind, P & Wolfer, W
Partner: UNT Libraries Government Documents Department

ALLOYING-DRIVEN PHASE STABILITY IN GROUP-VB TRANSITION METALS UNDER COMPRESSION

Description: The change in phase stability of Group-VB (V, Nb, and Ta) transition metals due to pressure and alloying is explored by means of first-principles electronic-structure calculations. It is shown that under compression stabilization or destabilization of the ground-state body-centered cubic (bcc) phase of the metal is mainly dictated by the band-structure energy that correlates well with the position of the Kohn anomaly in the transverse acoustic phonon mode. The predicted position of the Kohn anomaly in V, Nb, and Ta is found to be in a good agreement with data from the inelastic x-ray or neutron scattering measurements. In the case of alloying the change in phase stability is defined by the interplay between the band-structure and Madelung energies. We show that band-structure effects determine phase stability when a particular Group-VB metal is alloyed with its nearest neighbors within the same d-transition series: the neighbor with less and more d electrons destabilize and stabilize the bcc phase, respectively. When V is alloyed with neighbors of a higher (4d- or 5d-) transition series, both electrostatic Madelung and band-structure energies stabilize the body-centered-cubic phase. The opposite effect (destabilization) happens when Nb or Ta is alloyed with neighbors of the 3d-transition series.
Date: April 11, 2011
Creator: Landa, A & Soderlind, P
Partner: UNT Libraries Government Documents Department

Electronic Structure Calculations of delta-Pu Based Alloys

Description: First-principles methods are employed to study the ground-state properties of {delta}-Pu-based alloys. The calculations show that an alloy component larger than {delta}-Pu has a stabilizing effect. Detailed calculations have been performed for the {delta}-Pu{sub 1-c}Am{sub c} system. Calculated density of Pu-Am alloys agrees well with the experimental data. The paramagnetic {yields} antiferromagnetic transition temperature (T{sub c}) of {delta}-Pu{sub 1-c}Am{sub c} alloys is calculated by a Monte-Carlo technique. By introducing Am into the system, one could lower T{sub c} from 548 K (pure Pu) to 372 K (Pu{sub 70}Am{sub 30}). We also found that, contrary to pure Pu where this transition destabilizes {delta}-phase, Pu{sub 3}Am compound remains stable in the antiferromagnetic phase that correlates with the recent discovery of a Curie-Weiss behavior of {delta}-Pu{sub 1-c}Am{sub c} at c {approx} 24 at. %.
Date: November 13, 2003
Creator: Landa, A; Soderlind, P & Ruban, A
Partner: UNT Libraries Government Documents Department

Density-Functional Study of Magnetism in''Delta-Pu'' and its Alloys

Description: At atmospheric pressure plutonium metal exhibits six crystal structures upon heating from room temperature to its melting point of 913 K. Among these phases, the {delta}-Pu has received a significant interest in the metallurgical community because of its high ductility that makes it easy to machine and form. The pure Pu {delta} phase is stable at elevated temperatures, between 593 K and 736 K, however a small amount of so-called {delta} stabilizer, for example, Al, Ga, Zn, Cd, Zr, SC, Am, Ce, In, or Tl can help to retain {delta}-Pu down to ambient temperature. In recent papers we proposed that {delta}-Pu is a disordered magnet that becomes unstable below about 600 K due to the antiferromagnetic (AF) ordering followed by a mechanical instability of the FCC phase. In this paper we study the effect of alloying on the magnetic properties of {delta}-Pu and the stability of the FCC phase. There were several recent attempts to calculate electronic structure of Pu-based alloys. Becker et al. [3-5] have performed all electron full potential linear muffin-tin orbitals (FPLMTO) calculations of the electronic structure of Pu compounds (Pu{sub 3}X) with group-IIIB metals (X = Al, Ga, In and Tl). It is well known [6] that X-saturated {delta}-phase alloys forms two phase equilibrium with a Pu{sub 3}X compound that is isostructural to the Cu{sub 3}Au (Ll{sub 2}) structure and the stability of the {delta} phase depends upon stability of this compound. These calculations failed severely to reproduce the equilibrium lattice parameter of the Cu{sub 3}Au-type compounds resulting in so-called overbonding and overly large density. Moreover, the calculated formation energies of Pu{sub 3}In and Pu{sub 3}Tl compounds appear to be positive which does not correspond the reality. In order to improve results of these calculations Turchi et al. [7] used the phenomenological LDA+U method exploiting the ...
Date: September 30, 2002
Creator: Landa, A & Soderlind, P
Partner: UNT Libraries Government Documents Department

Density-functional calculations of a, b, g, d, dp, and e plutonium

Description: Total energies for the six known polymorphs of plutonium metal have been calculated within spin and orbital polarized density-functional theory as a function of lattice constant. Theoretical equilibrium volumes and bulk moduli correspond well with experimental data and the calculated total energies are consistent with the known phase diagram of Pu. It is shown that a preference for formation of magnetic moments, increasing through the {alpha} {yields} {beta} {yields} {gamma} phases, explain their position in the ambient pressure phase diagram and their anomalous variation of atomic density. A simple model is presented that establishes a relationship between atomic density, crystal symmetry, and magnetic moments which is universally valid for all Pu phases.
Date: October 22, 2003
Creator: Soderlind, P & Sadigh, B
Partner: UNT Libraries Government Documents Department

DENSITY-FUNCTIONAL CALCULATIONS FOR Ce, Th, AND Pu METALS AND ALLOYS

Description: The phase diagrams of Ce, Th, and Pu metals have been studied by means of density-functional theory (DFT). In addition to these metals, the phase stability of Ce-Th and Pu-Am alloys has been also investigated from first-principles calculations. Equation-of-state (EOS) for Ce, Th, and the Ce-Th alloys has been calculated up to 1 Mbar pressure in good comparison to experimental data. Present calculations shows that the Ce-Th alloys adopt a body-centeredtetragonal (bct) structure upon hydrostatic compression that is in excellent agreement with measurements. The ambient pressure phase diagram of Pu is shown to be very poorly described by traditional DFT but rather well modeled when including magnetic interactions. In particular, the anomalous {var_sigma} phase of Pu is shown to be stabilized by magnetic disorder at elevated temperatures. The Pu-Am system has also been studied in a similar fashion and it is shown that this system, for about 25% Am content, becomes antiferromagnetic below about 400 K that corroborate the recent discovery of a Curie-Weiss behavior in this system.
Date: April 5, 2004
Creator: Landa, A & Soderlind, P
Partner: UNT Libraries Government Documents Department

First-principles thermoelasticity of transition metals at high pressure I. Tantalum prototype in the quasi-harmonic limit

Description: The thermoelastic properties of bcc tantalum have been investigated over a broad range of pressures (up to 10 Mbar) and temperatures (up to 26,000 K) using a new first-principles approach that accurately accounts for cold, electron-thermal, and ion-thermal contributions in materials where anharmonic effects are small. Specifically, we have combined ab initio full-potential linear-muffin-tin-orbital (FP-LMTO) electronic-structure calculations for the cold and electron-thermal contributions to the elastic moduli with phonon contributions for the ion-thermal part calculated using model generalized pseudopotential theory (MGPT). For the latter, a summation of terms over the Brillouin zone is performed within the quasi-harmonic approximation, where each term is composed of a strain derivative of the phonon frequency at a particular k point. At ambient pressure, the resulting temperature dependence of the Ta elastic moduli is in excellent agreement with ultrasonic measurements. The experimentally observed anomalous behavior of C{sub 44} at low temperatures is shown to originate from the electron-thermal contribution. At higher temperatures, the main contribution to the temperature dependence of the elastic moduli comes from thermal expansion, but inclusion of the electron- and ion-thermal contributions is essential to obtain quantitative agreement with experiment. In addition, the pressure dependence of the moduli at ambient temperature compares well with recent diamond-anvil cell measurements to 1.05 Mbar. Moreover, the calculated longitudinal and bulk sound velocities in polycrystalline Ta at higher pressure and temperature in the vicinity of shock melting ({approx} 3 Mbar) agree well with data obtained from shock experiments. However, at high temperatures along the melt curve above 1 Mbar, the B{prime} shear modulus becomes negative indicating the onset of unexpectedly strong anharmonic effects. Finally, the assumed temperature dependence of the Steinberg-Guinan strength model obtained from scaling with the bulk shear modulus is examined at ambient pressure.
Date: April 25, 2006
Creator: Orlikowski, D; Soderlind, P & Moriarty, J A
Partner: UNT Libraries Government Documents Department

DENSITY-FUNCTIONAL STUDY OF U-Mo AND U-Zr ALLOYS

Description: Density-functional theory previously used to describe phase equilibria in U-Zr alloys [A. Landa, P. Soederlind, P.E.A. Turchi, J. Alloys Comp. 478 (2009) 103-110] is extended to investigate the ground-state properties of U-Mo solid solutions. We discuss how the heat of formation in both alloys correlates with the charge transfer between the alloy components, and how the specific behavior of the density of states in the vicinity of the Fermi level promotes the stabilization of the U{sub 2}Mo compound. Our calculations prove that, due to the existence of a single {gamma}-phase over the typical fuel operation temperatures, {gamma}-U-Mo alloys should indeed have much lower constituent redistribution than {gamma}-U-Zr alloys for which binodal decomposition causes a high degree of constituent redistribution.
Date: November 1, 2010
Creator: Landa, A; Soderlind, P & Turchi, P A
Partner: UNT Libraries Government Documents Department