Alexander Landa

Fermi surface nesting and pre-martensitic softening in V and Nb at high pressures

First-principles total-energy calculations were performed for the trigonal shear elastic constant (C44) of body-centred cubic (bcc) V and Nb. A mechanical instability in C44 is found for V at pressures of ~2 Mbar which also shows a softening in Nb at pressures of ~0.5 Mbar. We argue that the pressure-induced shear instability (softening) of V (Nb) is due to the intra-band nesting of the Fermi surface.

On the electronic configuration in Pu: spectroscopy and theory

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 5fn occupation for Pu of 5≤n<6, with , 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, x-ray absorption, and electron energy loss spectra. Instead, these spectra support the traditional conjecture of a 5f5 occupation 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.

Phonon and magnetic structure in δ-plutonium from density-functional theory

We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure and (ii) the disordered-local-moment method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, but the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments.

Origin of the multiple configurations that drive the response of δ-plutonium’s elastic moduli to temperature

Significance Our measurements have shown that the face-centered cubic (δ)-phase of plutonium exhibits little or no volume change but softens drastically as temperature rises over a very broad range. This behavior constrains any microscopic theory of plutonium to require that more than one thermodynamically accessible configuration exists, that the higher energy one has a smaller volume, and contrary to most models, the higher energy, smaller volume configuration must be more compressible. Until the recent neutron-scattering discovery of dynamic magnetism in plutonium, no evidence for such configurations existed. With that discovery, we are able to justify our microscopic theory that captures the observed neutron-scattering and temperature behavior of δ-plutonium. 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 activated spin fluctuations.

Zero-Kelvin Compression Isotherms of the Elements 1 ≤ Z ≤ 92 to 100 GPa

Most of the chemical elements have now been compressed close to or above 100 GPa (1 Mbar) pressure in diamond-anvil cells and the pressure–volume room-temperature isotherms have been measured. We collect these data and use simple lattice-dynamics models to reduce the isotherms to 0 K. We have extended the published work by making new diamond-anvil-cell measurements on Cr and Rh, and by conducting density-functional calculations on the elements Po, At, Rn, Fr, Ra, and Ac. The 0 K data are tabulated for all elements 1 ≤ Z ≤ 92 and 0 ≤ P ≤ 100 GPa. These data are useful for generating wide-range equation of state models and for studying the stability of chemical compounds at high pressure (“Megabar chemistry”). The tables presented here are intended to be reference thermodynamic tables for use in high-pressure research. Further experimental and theoretical work will be needed to extend the tables to higher pressure and to improve accuracy.

Reply to Janoschek et al.: The excited δ-phase of plutonium

In a recent PNAS paper (1) we explain the anomalous temperature dependence of the elasticity in δ-plutonium (δ-Pu) in terms of a first-principles model that includes multiple energy configurations attributed to spin fluctuations. Our model (1) captures the highly unusual behavior of decreasing elastic moduli with increasing temperature at constant volume. This is a falsifiable test that any model must pass but none, other than the one we present (1), has. This test is a difficult one, as usually the smaller volume state required to keep volume constant is elastically stiffer, contrary to measurements. Our theory yields a modest decrease by about … [↵][1]1To whom correspondence should be addressed. Email: migliori{at}lanl.gov. [1]: #xref-corresp-1-1

Prediction of the new efficient permanent magnet SmCoNiFe3

We propose a new efficient permanent magnet, SmCoNiFe3, which is a development of the well-known SmCo5 prototype. More modern neodymium magnets of the Nd-Fe-B type have an advantage over SmCo5 beca ...

Elastic moduli of

We study the time evolution (aging) of the elastic moduli of an eight-year-old polycrystalline δ-Pu 2.0 at. % Ga alloy (δ-Pu:Ga) at different fixed temperatures from 295 K to nearly 500 K in real time using Resonant Ultrasound Spectroscopy. After 8 years of aging at 295 K, the bulk and shear moduli increase at a normalized rate of 0.2%/year and 0.6%/year, respectively. As the temperature is raised, two time dependences are observed, an exponential one of about a week, followed by a linear one (constant rate). The linear rate is thermally activated with an activation energy of 0.33 + 0.06 eV. Above 420 K a qualitative change in the time evolution is observed; the bulk modulus decreases with time while the shear modulus continues to stiffen. No change in the behavior of the time evolution is observed as the α−β transition temperature is crossed as would be expected if a decomposition of δ-Pu:Ga to α-Pu and Pu3Ga occurred over the temperature range studied. Our results indicate that the main mechanism of agi...

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In the periodic table, only a few pure metals exhibit lattice or magnetic instabilities associated with Fermi surface nesting, the classical examples being α-U and Cr. Whereas α-U displays a strong Kohn anomaly in the phonon spectrum that ultimately leads to the formation of charge density waves (CDWs), Cr is known for its nesting-induced spin density waves (SDWs). Recently, it has become clear that a pronounced Kohn anomaly and the corresponding softening in the elastic constants is also the key factor that controls structural transformations and mechanical properties in compressed group VB metals—materials with relatively high superconducting critical temperatures. This article reviews the current understanding of the structural and mechanical behavior of these metals under pressure with an introduction to the concept of the Kohn anomaly and how it is related to the important concept of Peierls instability. We review both experimental and theoretical results showing different manifestations of the Kohn anomaly in the transverse acoustic phonon mode TA (ξ00) in V, Nb, and Ta. Specifically, in V the anomaly triggers a structural transition to a rhombohedral phase, whereas in Nb and Ta it leads to an anomalous reduction in yield strength.

-Pu239 reveal aging in real time

The change in phase stability of Group-VB transition metals (V, Nb, and Ta) 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. 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 bcc phase. Utilizing the self-consistent ab initio lattice dynamics approach, we show that pressure-induced mechanical instability of bcc V, which results in formation of a rhombohedral (rh) phase at around 60-70 GPa at room temperatures, will prevail significant heating and compression. Furthermore, alloying with Cr decreases the temperature at which stabilization of the bcc phase occurs at elevated pressure.