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Atomistic simulations for multiscale modeling in bcc metal

Description: Quantum-based atomistic simulations are being used to study fundamental deformation and defect properties relevant to the multiscale modeling of plasticity in bcc metals at both ambient and extreme conditions. Ab initio electronic-structure calculations on the elastic and ideal-strength properties of Ta and Mo help constrain and validate many-body interatomic potentials used to study grain boundaries and dislocations. The predicted C(capital Sigma)5 (310)[100] grain boundary structure for Mo has recently been confirmed in HREM measurements. The core structure, (small gamma) surfaces, Peierls stress, and kink-pair formation energies associated with the motion of a/2(111) screw dislocations in Ta and Mo have also been calculated. Dislocation mobility and dislocation junction formation and breaking are currently under investigation.
Date: September 25, 1998
Creator: Belak, J.; Moriarty, J.A.; Soderlind, P.; Xu, W.; Yang, L.H. & Zhu
Partner: UNT Libraries Government Documents Department

Delocalization and new phase in Americium: theory

Description: Density-functional electronic structure calculations have been used to investigate the high pressure behavior of Am. At about 80 kbar (8 GPa) calculations reveal a monoclinic phase similar to the ground state structure of plutonium ({alpha}-Pu). The experimentally suggested {alpha}-U structure is found to be substantially higher in energy. The phase transition from fcc to the low symmetry structure is shown to originate from a drastic change in the nature of the electronic structure induced by the elevated pressure. A calculated volume collapse of about 25% is associated with the transition. For the low density phase, an orbital polarization correction to the local spin density (LSD) theory was applied. Gradient terms of the electron density were included in the calculation of the exchange/correlation energy and potential, according to the generalized gradient approximation (GGA). The results are consistent with a Mott transition; the 5f electrons are delocalized and bonding on the high density side of the transition and chemically inert and non-bonding (localized) on the other. Theory compares rather well with recent experimental data which implies that electron correlation effects are reasonably modeled in our orbital polarization scheme.
Date: April 23, 1999
Creator: Soderlind, P
Partner: UNT Libraries Government Documents Department

First Principles Thermoelasticity of Tantalum at High Pressures

Description: The thermoelastic properties of bcc tantalum have been investigated over a broad range of temperatures (up to 12000 K) and pressures (up to 10 Mbar) using first-principles methods that account for cold, electron-thermal, and ion-thermal contributions. Specifically, we have combined ab initio all electron 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 elastic moduli is in excellent agreement with ultrasonic measurements. The experimentally observed anomalous behavior of C44 at low temperatures is shown to originate from the electron-thermal contribution. At higher temperatures, the dominant contribution to the temperature dependence of the elastic moduli comes from thermal expansion. Also, the pressure dependence of the moduli compares well with recent diamond and cell measurements up to 105 GPa. The calculated longitudinal and bulk sound velocities at higher pressure and temperature agree well with data obtained from shock experiments. Additionally, the temperature dependence of the Steinberg-Guinan model is examined for ambient pressure.
Date: June 21, 2002
Creator: Orlikowski, D.A.; Soderlind, P. & Moriarty, J.
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

AB INITIO CALCULATIONS OF ELASTIC CONSTANTS OF BCC V-NB SYSTEM AT HIGH PRESSURES

Description: First-principles total energy calculation based on the exact muffin-tin orbital and full potential linear muffin-tin orbital methods were used to calculate the equation of state and shear elastic constants of bcc V, Nb, and the V{sub 95}Nb{sub 05} disordered alloy as a function of pressure up to 6 Mbar. We found a mechanical instability in C{sub 44} and a corresponding softening in C at pressures {approx} 2 Mbar for V. Both shear elastic constants show softening at pressures {approx} 0.5 Mbar for Nb. Substitution of 5 at. % of V with Nb removes the instability of V with respect to trigonal distortions in the vicinity of 2 Mbar pressure, but still leaves the softening of C{sub 44} in this pressure region. We argue that the pressure induced shear instability (softening) of V (Nb) originates from the electronic system and can be explained by a combination of the Fermi surface nesting, electronic topological transition, and band Jahn-Teller effect.
Date: May 2, 2005
Creator: Landa, A; Klepeis, J; Soderlind, P; Naumov, I; Velikokhatnyi, O; Vitos, L et al.
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

Quantum-based Atomistic Simulation of Transition Metals

Description: First-principles generalized pseudopotential theory (GPT) provides a fundamental basis for transferable multi-ion interatomic potentials in d-electron transition metals within density-functional quantum mechanics. In mid-period bcc metals, where multi-ion angular forces are important to structural properties, simplified model GPT or MGPT potentials have been developed based on canonical d bands to allow analytic forms and large-scale atomistic simulations. Robust, advanced-generation MGPT potentials have now been obtained for Ta and Mo and successfully applied to a wide range of structural, thermodynamic, defect and mechanical properties at both ambient and extreme conditions of pressure and temperature. Recent algorithm improvements have also led to a more general matrix representation of MGPT beyond canonical bands allowing increased accuracy and extension to f-electron actinide metals, an order of magnitude increase in computational speed, and the current development of temperature-dependent potentials.
Date: August 29, 2005
Creator: Moriarty, J A; Benedict, L X; Glosli, J N; Hood, R Q; Orlikowski, D A; Patel, M V et al.
Partner: UNT Libraries Government Documents Department

Robust Quantum-Based Interatomic Potentials for Multiscale Modeling in Transition Metals

Description: First-principles generalized pseudopotential theory (GPT) provides a fundamental basis for transferable multi-ion interatomic potentials in transition metals and alloys within density-functional quantum mechanics. In the central bcc metals, where multi-ion angular forces are important to materials properties, simplified model GPT or MGPT potentials have been developed based on canonical d bands to allow analytic forms and large-scale atomistic simulations. Robust, advanced-generation MGPT potentials have now been obtained for Ta and Mo and successfully applied to a wide range of structural, thermodynamic, defect and mechanical properties at both ambient and extreme conditions. Selected applications to multiscale modeling discussed here include dislocation core structure and mobility, atomistically informed dislocation dynamics simulations of plasticity, and thermoelasticity and high-pressure strength modeling. Recent algorithm improvements have provided a more general matrix representation of MGPT beyond canonical bands, allowing improved accuracy and extension to f-electron actinide metals, an order of magnitude increase in computational speed for dynamic simulations, and the development of temperature-dependent potentials.
Date: September 27, 2005
Creator: Moriarty, J A; Benedict, L X; Glosli, J N; Hood, R Q; Orlikowski, D A; Patel, M V et al.
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

Symmetry reduction of (delta)-plutonium: an electronic-structure effect

Description: Using first-principles density-functional theory calculations, we show that the anomalously large anisotropy of {sigma}-plutonium is a consequence of greatly varying bond-strengths between the 12 nearest neighbors. Employing the calculated bond strengths, we expand the tenants of classical crystallography by incorporating anisotropy of chemical bonds, which yields a structure with the monoclinic space group Cm for {delta}-plutonium rather than face-centered cubic Fm{bar 3}m. The reduced space group for {delta}-plutonium enlightens why the ground state of the metal is monoclinic, why distortions of the metal are viable, and has considerable implications for the behavior of the material as it ages. These results illustrate how an expansion of classical crystallography that accounts for anisotropic electronic structure can explain complicated materials in a novel way.
Date: November 16, 2005
Creator: Moore, K; Soderlind, P; Schwartz, A & Laughlin, D
Partner: UNT Libraries Government Documents Department

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

FERMI SURFACE NESTING AND PRE-MARTENSITIC SOFTENING IN V AND Nb AT HIGH PRESSURES

Description: First-principles total-energy calculations were performed for the trigonal shear elastic constant (C{sub 44}) of vanadium and niobium. A mechanical instability in C{sub 44} is found for vanadium at pressures {approx} 2 Mbar which also shows softening in niobium at pressures {approx} 0.5 Mbar. We argue that the pressure-induced shear instability (softening) of vanadium (niobium) is due to the intraband nesting of the Fermi surface.
Date: December 21, 2005
Creator: Landa, A; Klepeis, J; Soderlind, P; Naumov, I; Velikokhatnyi, O; Vitos, L et al.
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

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

On the electronic configuration in Pu: spectroscopy and theory

Description: Photoelectron spectroscopy, synchrotron-radiation-based x-ray absorption, electron energy-loss spectroscopy, and density-functional calculations within the mixed-level and magnetic models, together with canonical band theory have been used to study the electron configuration in Pu. These methods suggest a 5f{sup n} configuration for Pu of 5 {le} n < 6, with n {ne} 6, contrary to what has recently been suggested in several publications. We show that the n = 6 picture is inconsistent with the usual interpretation of photoemission and x-ray absorption spectra. Instead, these spectra support the traditional conjecture of a 5f{sup 5} configuration in Pu as is obtained by density-functional theory. We further argue, based on 5f-band filling, that an n = 6 hypothesis is incompatible with the position of Pu in the actinide series and its monoclinic ground-state phase.
Date: October 11, 2006
Creator: Tobin, J G; Soderlind, P; Landa, A; Moore, K T; Schwartz, A J; Chung, B W et al.
Partner: UNT Libraries Government Documents Department

Capabilities for Testing the Electronic Configuration in Pu

Description: The benchmarking of theoretical modeling is crucial to the ultimate determination of the nature of the electronic structure of Pu. Examples of experimental techniques used for cross checking state of the art calculations will be given.
Date: November 8, 2006
Creator: Tobin, J G; Soderlind, P; Landa, A; Moore, K T; Schwartz, A J; Chung, B W et al.
Partner: UNT Libraries Government Documents Department

Emergence of Strong Exchange Interaction in the Actinide Series: The Driving Force for Magnetic Stabilization of Curium

Description: Using electron energy-loss spectroscopy in a transmission electron microscope, many-electron atomic spectral calculations and density functional theory, we examine the electronic and magnetic structure of Cm metal. We show that angular momentum coupling in the 5f states plays a decisive role in the formation of the magnetic moment. The 5f states of Cm in intermediate coupling are strongly shifted towards the LS coupling limit due to exchange interaction, unlike most actinide elements where the effective spin-orbit interaction prevails. It is this LS-inclined intermediate coupling that is the key to producing the large spin polarization which in turn dictates the newly found crystal structure of Cm under pressure.
Date: January 4, 2007
Creator: Moore, K; der Laan, G v; Haire, D; Wall, M; Schwartz, A & Soderlind, P
Partner: UNT Libraries Government Documents Department

Atomic-volume variations of (alpha)-Pu alloyed with Al, Ga, and Am from first-principles theory

Description: First-principles methods are employed to calculate the ground-state atomic densities (or volumes) of {alpha}-Pu alloyed with Al, Ga, and Am. Three configurations for the alloying atom are considered. (1) It is located at the most open and energetically most favorably site. (2) It is located in the least open site. (3) It is randomly distributed within the {alpha}-Pu matrix. When alloyed with Al or Ga, {alpha}-Pu behaves similarly, it expands considerably for configurations (2) and (3), while for (1) only small changes of the density occurs. Interestingly, for Am the alloying effects are quite different from that of Al and Ga. Small expansion is noted for the ordered configurations (1) and (2), whereas for the disordered (3), only insignificant changes of the density take place. The bonding character is thus differently influenced in Pu by the addition of Al and Ga on one hand and Am on the other. This is consistent with the view that Al and Ga stabilize the {delta} over the {alpha} phase in Pu by a different mechanism than Am, as has been discussed in recent publications.
Date: January 9, 2007
Creator: Soderlind, P; Landa, A & Wolfer, W G
Partner: UNT Libraries Government Documents Department

On The Electronic Configuration in Pu

Description: X-Ray Absorption Spectroscopy (XAS) and Photoelectron Spectroscopy (PES) have been performed upon highly radioactive samples, particularly Plutonium, at the Advanced Light Source in Berkeley, CA, USA. First results from alpha and delta Plutonium are reported as well as a detailed analysis of sample quality.
Date: January 29, 2007
Creator: Tobin, J G; Soderlind, P; Landa, A; Moore, K T; Schwartz, A J; Chung, B W et al.
Partner: UNT Libraries Government Documents Department

Theoretical confirmation of a high-pressure rhombohedral phase in vanadium metal

Description: Recent diamond-anvil-cell (DAC) experiments revealed a new phase in vanadium metal at high pressure. Here we present results from first-principles electronic-structure calculations confirming the existence of such phase. The new phase is due to a rhombohedral distortion of the body-centered-cubic (bcc) ambient-pressure phase. The calculated transition pressure of 0.84 Mbar and density compare favorably with the measured data. Interestingly, a re-entrant bcc phase is discovered at an ultra high pressure, close to the limit of DAC experimental capabilities, of about 2.8 Mbar. We show, extending prior work, that the phase transitions in vanadium are driven by subtle electronic-structure effects.
Date: February 27, 2007
Creator: Lee, B; Rudd, R E; Klepeis, J; Soderlind, P & Landa, A
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

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