Alexander Landa | Lawrence Livermore National Lab (original) (raw)
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Papers by Alexander Landa
Applied Sciences
Thermodynamics of plutonium monocarbide is studied from first-principles theory that includes rel... more Thermodynamics of plutonium monocarbide is studied from first-principles theory that includes relativistic electronic structure and anharmonic lattice vibrations. Density-functional theory (DFT) is expanded to include orbital-orbital coupling in addition to the relativistic spin-orbit interaction for the electronic structure and it is combined with anharmonic, temperature dependent, lattice dynamics derived from the self-consistent ab initio lattice dynamics (SCAILD) method. The obtained thermodynamics are compared to results from simpler quasi-harmonic theory and experimental data. Formation enthalpy, specific heat, and Gibbs energy calculated from the anharmonic model are validated by direct comparison with a calculation of phase diagram (CALPHAD) assessment of PuC and sub-stochiometric PuC0.896. Overall, the theory reproduces CALPHAD results and measured data for PuC rather well, but the comparison is hampered by the sub-stoichiometric nature of plutonium monocarbide. It was foun...
Applied Sciences
The interaction of actinides and actinide alloys such as the δ-stabilized Pu-Ga alloy with iron i... more The interaction of actinides and actinide alloys such as the δ-stabilized Pu-Ga alloy with iron is of interest to understand the impurity effects on phase stability. A newly developed and self-consistent CALPHAD thermodynamic database is presented which covers the elements: Pu, U, Fe, Ga across their whole composition and temperature ranges. The phase diagram and thermodynamic properties of plutonium-iron (Pu-Fe) and uranium-iron (U-Fe) systems are successfully reassessed, with emphasis on the actinide rich side. Density functional theory (DFT) calculations are performed to validate the stability of the stoichiometric (Pu,U)6Fe and (Pu,U)Fe2 compounds by computing their formation enthalpies. These data are combined to construct the Pu-U-Fe ternary phase diagram. The thermodynamic assessment of Fe-Ga is presented for the first time and application to the quaternary Pu-U-Fe-Ga system is discussed.
Applied Sciences
First-principles calculations within the density-functional-theory (DFT) approach are conducted i... more First-principles calculations within the density-functional-theory (DFT) approach are conducted in order to explore and explain the effect of small amounts of titanium on phase stability in the U-6Nb alloy. During rapid quenching from high to room temperature, metastable phases α′ (orthorhombic), α″ (monoclinic), and γ0 (tetragonal) can form, depending on Nb concentration. Important mechanical properties depend on the crystal structure and, therefore, an understanding of the effect of impurities on phase stability is essential. Insights on this issue are obtained from quantum-mechanical DFT calculations. The DFT framework does not rely on any material-specific assumptions and is therefore ideal for an unbiased investigation of the U-Nb system.
Appl. Sci., 2020
The interaction of actinides and actinide alloys such as the δ-stabilized Pu-Ga alloy with iron i... more The interaction of actinides and actinide alloys such as the δ-stabilized Pu-Ga alloy with iron is of interest to understand the impurity effects on phase stability. A newly developed and self-consistent CALPHAD thermodynamic database is presented which covers the elements: Pu, U, Fe, Ga across their whole composition and temperature ranges. The phase diagram and thermodynamic properties of plutonium-iron (Pu-Fe) and uranium-iron (U-Fe) systems are successfully reassessed, with emphasis on the actinide rich side. Density functional theory (DFT) calculations are performed to validate the stability of the stoichiometric (Pu,U) 6 Fe and (Pu,U)Fe 2 compounds by computing their formation enthalpies. These data are combined to construct the Pu-U-Fe ternary phase diagram. The thermodynamic assessment of Fe-Ga is presented for the first time and application to the quaternary Pu-U-Fe-Ga system is discussed.
Materials & Design
Abstract Producing gram quantities of uranium metal in a controlled manner by traditional methods... more Abstract Producing gram quantities of uranium metal in a controlled manner by traditional methods is challenging due to the complex chemistry of precursor material and extreme thermal requirements. In this article, a novel approach is reported that combines modeling and an advanced experimental technique for extracting uranium from a uranium-containing compound. Using uranium nitride as an example, a computational thermodynamic approach identified a decomposition pathway to convert uranium nitride to uranium metal at temperatures exceeding 2500 K under conditions of rapid material cooling. To realize these extreme conditions, laser-induced heating, which enables fine control of process location and rapid cooling, was utilized for high-temperature modification of material. Uranium nitride was irradiated by a controlled laser under several gaseous conditions including high-vacuum, argon, and nitrogen environments, resulting in uranium metal at yields up to 96%. The complete decomposition leading to pure uranium metal occurs at the high temperature surface region, where laser-based heating induces a surface depression and molten pool of material. The observed kinetic phase behaviors in this study fundamentally differ from previous uranium decomposition studies where small uranium metal precipitates from the nitride bulk are formed at the surface of uranium nitride.
Applied Sciences
Density functional theory (DFT) calculations are employed to explore and assess the effects of th... more Density functional theory (DFT) calculations are employed to explore and assess the effects of the relativistic spin–orbit interaction and electron correlations in the actinide elements. Specifically, we address electron correlations in terms of an intra-atomic Coulomb interaction with a Hubbard U parameter (DFT + U). Contrary to recent beliefs, we show that for the ground-state properties of the light actinide elements Th to Pu, the DFT + U makes its best predictions for U = 0. Actually, our modeling suggests that the most popular DFT + U formulation leads to the wrong ground-state phase for plutonium. Instead, extending DFT and the generalized gradient approximation (GGA) with orbital–orbital interaction (orbital polarization; OP) is the most accurate approach. We believe the confusion in the literature on the subject mostly originates from incorrectly accounting for the spin–orbit (SO) interaction for the p1/2 state, which is not treated in any of the widely used pseudopotential ...
Applied Sciences
We report on an advanced density-functional theory (DFT) approach for investigating the ground-st... more We report on an advanced density-functional theory (DFT) approach for investigating the ground-state and thermodynamical properties of uranium mononitride (UN). The electronic structure for UN at zero temperature is obtained from DFT that utilizes the generalized gradient approximation (GGA) for the electron exchange and correlation functional and includes spin-orbit interaction and an extension with orbital polarization. Thermodynamical properties are computed within the quasi-harmonic approximation in the Debye–Grüneisen model while anharmonicity is captured in the self-consistent ab initio lattice dynamics (SCAILD) scheme. Anharmonic phonons have heretofore never been modeled from first-principles for UN but they turn out to be important. The computed free energy compares well with that of a CALPHAD (CALculation of PHAse Diagrams) assessment of available experimental data.
Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences, 2016
The electronic and thermodynamic complexity of plutonium has resisted a fundamental understanding... more The electronic and thermodynamic complexity of plutonium has resisted a fundamental understanding for this important elemental metal. A critical test of any theory is the unusual softening of the bulk modulus with increasing temperature, a result that is counterintuitive because no or very little change in the atomic volume is observed upon heating. This unexpected behavior has in the past been attributed to competing but never-observed electronic states with different bonding properties similar to the scenario with magnetic states in Invar alloys. Using the recent observation of plutonium dynamic magnetism, we construct a theory for plutonium that agrees with relevant measurements by using density-functional-theory (DFT) calculations with no free parameters to compute the effect of longitudinal spin fluctuations on the temperature dependence of the bulk moduli in δ-Pu. We show that the softening with temperature can be understood in terms of a continuous distribution of thermally a...
J Phys Condens Matter, 2003
J Phys Condens Matter, 2003
... Alex Landa a , Corresponding Author Contact Information , E-mail The Corresponding Author , P... more ... Alex Landa a , Corresponding Author Contact Information , E-mail The Corresponding Author , Per Söderlind a and Levente Vitos b , c. ... Recently, Sadigh and Wolfer [3] performed plane-wave pseudopotential calculations for a variety of super-cell configurations based on the α ...
ABSTRACT We present results of ab initio calculations of equation-of-state and elastic properties... more ABSTRACT We present results of ab initio calculations of equation-of-state and elastic properties for Pu metal and Pu-Ga alloys. For this we have employed density-functional theory (DFT) in conjunction with spin-orbit coupling and orbital polarization for the metal and coherent-potential approximation (CPA) for the alloys. All Pu systems benefit from spin polarization which is consistent with previous DFT studies of plutonium. We show that orbital correlations become more important proceeding from alpha->beta->gamma plutonium, thus suggesting increasing f-electron correlation (localization). For delta-Pu- Ga alloys we find that the system softens with larger Ga content, i.e., bulk modulus, elastic constants, and chemical bonding weakens with increasing Ga concentration. This inverse relationship is nearly linear and supported by measurements on polycrystal delta-Pu-Ga alloys. For Pu metal, our single-crystal results also relates reasonably with ultrasound data on polycrystal samples where available. The comparison is indirect but made possible by approximating the polycrystal with an isotropic (uniform strain) single crystal. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Model Simul Mater Sci Eng, 2003
First-principles methods are employed to study the effect of localization of the 5f electrons in ... more First-principles methods are employed to study the effect of localization of the 5f electrons in dgr-Pu. First, a full-potential linear muffin-tin orbitals (FPLMTO) method was applied to a model system, Pu^loc_{x}Pu^it_{1-x} , where Puloc are Pu atoms with localized (nonbonding) 5f electrons and Puit atoms with itinerant (bonding) 5f electrons. Within the FPLMTO, this system was treated as an ordered compound, either in the Cu3Au or the CuAu structure to model dgr-Pu which crystallize in a face-centred-cubic structure. A more realistic alloy treatment of our model system was provided by the Korringa-Kohn-Rostocker method within Green's function formalism in which compositional disorder is treated by means of the coherent potential approximation. With these two approaches best agreement with the experimental lattice constant for dgr-Pu were achieved for a 67-68% fraction of itinerant (Puit) atoms. This corresponds to a little less than four itinerant 5f electrons/atom in dgr-Pu which agrees well with some proposed theoretical models, but disagree with at least an other theoretical suggestion. We show that a good lattice constant (by construction), good bulk modulus, and full mechanical stability for dgr-Pu follows from our model. The main problem with the present approach and some other presented models, trying to capture localization in dgr-Pu, is that the contribution to the total energy from the localized 5f electrons cannot be calculated accurately and therefore one parameter (usually the lattice constant) needs to be fitted to experiment.
Applied Sciences
Thermodynamics of plutonium monocarbide is studied from first-principles theory that includes rel... more Thermodynamics of plutonium monocarbide is studied from first-principles theory that includes relativistic electronic structure and anharmonic lattice vibrations. Density-functional theory (DFT) is expanded to include orbital-orbital coupling in addition to the relativistic spin-orbit interaction for the electronic structure and it is combined with anharmonic, temperature dependent, lattice dynamics derived from the self-consistent ab initio lattice dynamics (SCAILD) method. The obtained thermodynamics are compared to results from simpler quasi-harmonic theory and experimental data. Formation enthalpy, specific heat, and Gibbs energy calculated from the anharmonic model are validated by direct comparison with a calculation of phase diagram (CALPHAD) assessment of PuC and sub-stochiometric PuC0.896. Overall, the theory reproduces CALPHAD results and measured data for PuC rather well, but the comparison is hampered by the sub-stoichiometric nature of plutonium monocarbide. It was foun...
Applied Sciences
The interaction of actinides and actinide alloys such as the δ-stabilized Pu-Ga alloy with iron i... more The interaction of actinides and actinide alloys such as the δ-stabilized Pu-Ga alloy with iron is of interest to understand the impurity effects on phase stability. A newly developed and self-consistent CALPHAD thermodynamic database is presented which covers the elements: Pu, U, Fe, Ga across their whole composition and temperature ranges. The phase diagram and thermodynamic properties of plutonium-iron (Pu-Fe) and uranium-iron (U-Fe) systems are successfully reassessed, with emphasis on the actinide rich side. Density functional theory (DFT) calculations are performed to validate the stability of the stoichiometric (Pu,U)6Fe and (Pu,U)Fe2 compounds by computing their formation enthalpies. These data are combined to construct the Pu-U-Fe ternary phase diagram. The thermodynamic assessment of Fe-Ga is presented for the first time and application to the quaternary Pu-U-Fe-Ga system is discussed.
Applied Sciences
First-principles calculations within the density-functional-theory (DFT) approach are conducted i... more First-principles calculations within the density-functional-theory (DFT) approach are conducted in order to explore and explain the effect of small amounts of titanium on phase stability in the U-6Nb alloy. During rapid quenching from high to room temperature, metastable phases α′ (orthorhombic), α″ (monoclinic), and γ0 (tetragonal) can form, depending on Nb concentration. Important mechanical properties depend on the crystal structure and, therefore, an understanding of the effect of impurities on phase stability is essential. Insights on this issue are obtained from quantum-mechanical DFT calculations. The DFT framework does not rely on any material-specific assumptions and is therefore ideal for an unbiased investigation of the U-Nb system.
Appl. Sci., 2020
The interaction of actinides and actinide alloys such as the δ-stabilized Pu-Ga alloy with iron i... more The interaction of actinides and actinide alloys such as the δ-stabilized Pu-Ga alloy with iron is of interest to understand the impurity effects on phase stability. A newly developed and self-consistent CALPHAD thermodynamic database is presented which covers the elements: Pu, U, Fe, Ga across their whole composition and temperature ranges. The phase diagram and thermodynamic properties of plutonium-iron (Pu-Fe) and uranium-iron (U-Fe) systems are successfully reassessed, with emphasis on the actinide rich side. Density functional theory (DFT) calculations are performed to validate the stability of the stoichiometric (Pu,U) 6 Fe and (Pu,U)Fe 2 compounds by computing their formation enthalpies. These data are combined to construct the Pu-U-Fe ternary phase diagram. The thermodynamic assessment of Fe-Ga is presented for the first time and application to the quaternary Pu-U-Fe-Ga system is discussed.
Materials & Design
Abstract Producing gram quantities of uranium metal in a controlled manner by traditional methods... more Abstract Producing gram quantities of uranium metal in a controlled manner by traditional methods is challenging due to the complex chemistry of precursor material and extreme thermal requirements. In this article, a novel approach is reported that combines modeling and an advanced experimental technique for extracting uranium from a uranium-containing compound. Using uranium nitride as an example, a computational thermodynamic approach identified a decomposition pathway to convert uranium nitride to uranium metal at temperatures exceeding 2500 K under conditions of rapid material cooling. To realize these extreme conditions, laser-induced heating, which enables fine control of process location and rapid cooling, was utilized for high-temperature modification of material. Uranium nitride was irradiated by a controlled laser under several gaseous conditions including high-vacuum, argon, and nitrogen environments, resulting in uranium metal at yields up to 96%. The complete decomposition leading to pure uranium metal occurs at the high temperature surface region, where laser-based heating induces a surface depression and molten pool of material. The observed kinetic phase behaviors in this study fundamentally differ from previous uranium decomposition studies where small uranium metal precipitates from the nitride bulk are formed at the surface of uranium nitride.
Applied Sciences
Density functional theory (DFT) calculations are employed to explore and assess the effects of th... more Density functional theory (DFT) calculations are employed to explore and assess the effects of the relativistic spin–orbit interaction and electron correlations in the actinide elements. Specifically, we address electron correlations in terms of an intra-atomic Coulomb interaction with a Hubbard U parameter (DFT + U). Contrary to recent beliefs, we show that for the ground-state properties of the light actinide elements Th to Pu, the DFT + U makes its best predictions for U = 0. Actually, our modeling suggests that the most popular DFT + U formulation leads to the wrong ground-state phase for plutonium. Instead, extending DFT and the generalized gradient approximation (GGA) with orbital–orbital interaction (orbital polarization; OP) is the most accurate approach. We believe the confusion in the literature on the subject mostly originates from incorrectly accounting for the spin–orbit (SO) interaction for the p1/2 state, which is not treated in any of the widely used pseudopotential ...
Applied Sciences
We report on an advanced density-functional theory (DFT) approach for investigating the ground-st... more We report on an advanced density-functional theory (DFT) approach for investigating the ground-state and thermodynamical properties of uranium mononitride (UN). The electronic structure for UN at zero temperature is obtained from DFT that utilizes the generalized gradient approximation (GGA) for the electron exchange and correlation functional and includes spin-orbit interaction and an extension with orbital polarization. Thermodynamical properties are computed within the quasi-harmonic approximation in the Debye–Grüneisen model while anharmonicity is captured in the self-consistent ab initio lattice dynamics (SCAILD) scheme. Anharmonic phonons have heretofore never been modeled from first-principles for UN but they turn out to be important. The computed free energy compares well with that of a CALPHAD (CALculation of PHAse Diagrams) assessment of available experimental data.
Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences, 2016
The electronic and thermodynamic complexity of plutonium has resisted a fundamental understanding... more The electronic and thermodynamic complexity of plutonium has resisted a fundamental understanding for this important elemental metal. A critical test of any theory is the unusual softening of the bulk modulus with increasing temperature, a result that is counterintuitive because no or very little change in the atomic volume is observed upon heating. This unexpected behavior has in the past been attributed to competing but never-observed electronic states with different bonding properties similar to the scenario with magnetic states in Invar alloys. Using the recent observation of plutonium dynamic magnetism, we construct a theory for plutonium that agrees with relevant measurements by using density-functional-theory (DFT) calculations with no free parameters to compute the effect of longitudinal spin fluctuations on the temperature dependence of the bulk moduli in δ-Pu. We show that the softening with temperature can be understood in terms of a continuous distribution of thermally a...
J Phys Condens Matter, 2003
J Phys Condens Matter, 2003
... Alex Landa a , Corresponding Author Contact Information , E-mail The Corresponding Author , P... more ... Alex Landa a , Corresponding Author Contact Information , E-mail The Corresponding Author , Per Söderlind a and Levente Vitos b , c. ... Recently, Sadigh and Wolfer [3] performed plane-wave pseudopotential calculations for a variety of super-cell configurations based on the α ...
ABSTRACT We present results of ab initio calculations of equation-of-state and elastic properties... more ABSTRACT We present results of ab initio calculations of equation-of-state and elastic properties for Pu metal and Pu-Ga alloys. For this we have employed density-functional theory (DFT) in conjunction with spin-orbit coupling and orbital polarization for the metal and coherent-potential approximation (CPA) for the alloys. All Pu systems benefit from spin polarization which is consistent with previous DFT studies of plutonium. We show that orbital correlations become more important proceeding from alpha->beta->gamma plutonium, thus suggesting increasing f-electron correlation (localization). For delta-Pu- Ga alloys we find that the system softens with larger Ga content, i.e., bulk modulus, elastic constants, and chemical bonding weakens with increasing Ga concentration. This inverse relationship is nearly linear and supported by measurements on polycrystal delta-Pu-Ga alloys. For Pu metal, our single-crystal results also relates reasonably with ultrasound data on polycrystal samples where available. The comparison is indirect but made possible by approximating the polycrystal with an isotropic (uniform strain) single crystal. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Model Simul Mater Sci Eng, 2003
First-principles methods are employed to study the effect of localization of the 5f electrons in ... more First-principles methods are employed to study the effect of localization of the 5f electrons in dgr-Pu. First, a full-potential linear muffin-tin orbitals (FPLMTO) method was applied to a model system, Pu^loc_{x}Pu^it_{1-x} , where Puloc are Pu atoms with localized (nonbonding) 5f electrons and Puit atoms with itinerant (bonding) 5f electrons. Within the FPLMTO, this system was treated as an ordered compound, either in the Cu3Au or the CuAu structure to model dgr-Pu which crystallize in a face-centred-cubic structure. A more realistic alloy treatment of our model system was provided by the Korringa-Kohn-Rostocker method within Green's function formalism in which compositional disorder is treated by means of the coherent potential approximation. With these two approaches best agreement with the experimental lattice constant for dgr-Pu were achieved for a 67-68% fraction of itinerant (Puit) atoms. This corresponds to a little less than four itinerant 5f electrons/atom in dgr-Pu which agrees well with some proposed theoretical models, but disagree with at least an other theoretical suggestion. We show that a good lattice constant (by construction), good bulk modulus, and full mechanical stability for dgr-Pu follows from our model. The main problem with the present approach and some other presented models, trying to capture localization in dgr-Pu, is that the contribution to the total energy from the localized 5f electrons cannot be calculated accurately and therefore one parameter (usually the lattice constant) needs to be fitted to experiment.