Phonon dispersion of Mo-stabilized γ-U measured using inelastic x-ray scattering.

We have measured the room-temperature phonon spectrum of Mo-stabilized γ-U. The dispersion curves show unusual softening near the H point, q = [1/2, 1/2, 1/2], which may derive from the metastability of the γ-U phase or from strong electron-phonon coupling. Near the zone center, the dispersion curves agree well with theory, though significant differences are observed away from the zone center. The experimental phonon density of states is shifted to higher energy compared to theory and high-temperature neutron scattering. The elastic constants of γ-UMo are similar to those of body-centered cubic elemental metals.

Thermodynamic modeling of chromium: strong and weak magnetic coupling.

As chromium is a decisive ingredient for stainless steels, a reliable understanding of its thermodynamic properties is indispensable. Parameter-free first-principles methods have nowadays evolved to a state allowing such thermodynamic predictions. For materials such as Cr, however, the inclusion of magnetic entropy and higher order contributions such as anharmonic entropy is still a formidable task. Employing state-of-the-art ab initio molecular dynamics simulations and statistical concepts, we compute a set of thermodynamic properties based on quasiharmonic, anharmonic, electronic and magnetic free energy contributions from first principles. The magnetic contribution is modeled by an effective nearest-neighbor Heisenberg model, which itself is solved numerically exactly by means of a quantum Monte Carlo method. We investigate two different scenarios: a weak magnetic coupling scenario for Cr, as usually presumed in empirical thermodynamic models, turns out to be in clear disagreement with experimental observations. We show that instead a mixed Hamiltonian including weak and strong magnetic coupling provides a consistent picture with good agreement to experimental thermodynamic data.

Stability in bcc transition metals: Madelung and band-energy effects due to alloying.

The phase stability of group VB (V, Nb, and Ta) transition metals is explored by first-principles electronic-structure calculations. Alloying with a small amount of a neighboring metal can either stabilize or destabilize the body-centered-cubic phase relative to low-symmetry rhombohedral phases. 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. In this case, the neighbor with less (to the left) and more (to the right) d electrons destabilize and stabilize bcc, respectively. When alloying with neighbors of higher d-transition series, electrostatic Madelung energy dominates and stabilizes the body-centered-cubic phase. This surprising prediction invalidates current understanding of simple d-electron bonding that dictates high-symmetry cubic and hexagonal phases.

Emergence of strong exchange interaction in the actinide series: the driving force for magnetic stabilization of curium.

Using electron energy-loss spectroscopy, many-electron atomic spectral calculations, and density functional theory, we show that angular-momentum coupling in the 5f states plays a decisive role in the formation of the magnetic moment in Cm metal. 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. Hund's rule coupling is the key to producing the large spin polarization that dictates the newly found crystal structure of Cm under pressure.

Symmetry and stability of plutonium: the influence of electronic structure.

Using first-principles density-functional theory, we calculate the bond strengths between the 12 nearest neighbors in delta plutonium for both pure Pu and a Pu-3.7 at. % Ga alloy. Our results for pure Pu reveal a structure with the monoclinic space group Cm rather than face-centered cubic Fm3m, showing that the anomalously large anisotropy of delta plutonium is a consequence of greatly varying bond strengths between the 12 nearest neighbors. Further results for a Pu-3.7 at. % Ga alloy show that the nearest-neighbor bond strengths around a Ga atom are more uniform. Hence, our calculations address (i) why the ground state of Pu is monoclinic, (ii) why distortions of the delta phase are viable, with considerable implications for the behavior of the material as it ages due to anisotropic response to self-irradiation, and (iii) why Ga stabilizes face-centered cubic delta-Pu.

High-pressure melting curves of argon, krypton, and xenon: deviation from corresponding states theory.

The melting curves of argon, krypton, and xenon were measured in a laser heated diamond-anvil cell to pressures of nearly 80 GPa reaching melting temperatures around 3300 K. For the three gases, we observed a considerable decrease in the melting slopes (dT/dP) from the predictions based on corresponding states scaling starting near 40, 30, and 20 GPa, respectively. The melting anomaly can be understood in terms of a model in which hcp stacking faults act as solutes in a binary system.

New beta(fcc)-cobalt to 210 GPa

We report a new phase transition in cobalt from the magnetic varepsilon(hcp) to a beta(fcc) phase, likely nonmagnetic, at 105 GPa. It occurs martensitically in an extended pressure region between 105 and 150 GPa without any apparent volume change. The fcc phase of Co is in systematic accordance with the 4d and 5d counterparts. The pressure-volume isotherm of beta-Co resembles those of alpha(fcc)-Ni and varepsilon(hcp)-Fe within 1%. The phase diagram of cobalt suggests that the fcc stability increases with increasing occupancy of d-band electrons from Fe to Co to Ni.