High-temperature phonon stabilization ofγ-uranium from relativistic first-principles theory

Abstract

A microscopic explanation for temperature stabilization of the body-centered cubic (bcc) phase in the actinide metals is proposed. We show that for a prototype actinide, uranium, phonon-phonon interaction promotes bcc γ-U when heated even though at low temperatures it is mechanically a strongly unstable phase. Utilizing the recently developed self-consistent ab initio lattice dynamics (SCAILD) scheme in conjunction with highly accurate and fully relativistic density functional theory we obtain phonon dispersion and density of states that compare well with data acquired from inelastic neutron-scattering experiments. The investigation thus establishes that high-temperature lattice dynamics can be modeled from ab initio theory even for complex materials with substantial electron correlation including the actinides. PACS numbers:Density-functional theory (DFT) 1 has proven to be remarkably successful in describing the T = 0 K groundstate phases of most metals. Skriver 2 showed that it predicts the correct ground-state crystal structure for many transition elements when applied in a spherical linear muffin-tin orbital technique. Later 3 it became evident that DFT -implemented in more sophisticated methods appropriately treating complex crystal structures-also confirms the ground-state phases of the actinides. Except for metals with strong electron-correlation effects, such as the rare-earths, the DFT workhorse can nowadays be effectively used throughout the Periodic Table of Elements for low-temperature condensed-matter applications.There has been a long-standing difficulty, however, to model high-temperature phases with an accuracy analogous to that of the ground state at room or lower temperatures. The reason is that treating electronic and vibrational interactions simultaneously within a quantum-mechanical framework is a daunting task. It becomes particularly problematic when the high-temperature phase is mechanically unstable at low temperatures thus ruling out the typically applied perturbation theory of the zero-temperature electronic structure. An additional and potentially serious pitfall, when studying f -electron systems such as the actinide or rare-earth metals, is the possibility of a dramatic change in the f -electron behavior with temperature. For instance, cerium metal has a temperature-driven (α → γ) phase transition that has been argued to be caused by localization (Mott transition) 4 where the f states are removed from the Fermi level (highest occupied energy). Similarly, uranium (or any other actinide metal) could exhibit a localization of the 5f shell at elevated temperatures that cannot be addressed within conventional DFT. Localization is consistent with the formation of a high-symmetry phase such as bcc in U (see below) but because there is no significant volume expansion, often observed during this type of transition, we have reason to believe that this difficult-to-model mechanism is not present in this case. It was also shown that localization did not play a role in stabilizing bcc in Pu 5 ,...

Density-functional study of bcc U–Mo, Np–Mo, Pu–Mo, and Am–Mo alloys

“…The inaccuracy of the theoretical volume of -Np is consistent with previous calculations [42] and due to temperature effects of the actinide bcc phase that are difficult to model [43].…”

First-principles phase stability, bonding, and electronic structure of actinide metals

“…The agreement is satisfying at higher energies but there is a systematic discrepancy at lower temperatures. We discussed [50] that the difference may be due to the nonlinearity of some phonon branches approaching the Γ point apparent in Figure 10.…”

“…The method can be adopted for the actinide metals but the scheme requires ab initio forces on the thermally displaced atoms and these are difficult to compute, particularly for actinides. We have previously reported on the details of these calculations for uranium [50] that are more accurate in their method of obtaining atomic forces than previously proposed [49].…”

“…In spite of the difficulties to calculate forces accurately [50], we are applying SCAILD for γ-U and in Figure 10 we show the phonon dispersions at 1113 K. Notice that all branches are positive thus inferring full mechanical stability that is due to the phononphonon interactions accounted for within SCAILD [49]. This result implies that the bcc phase is likely not a result of strong electron correlations simply because a weakly correlated model (DFT-SCAILD) is sufficient to achieve the stabilization.…”

Thermodynamic properties and phase transitions of γ and liquid uranium: QMD and classical MD modeling

“…To this date both classical (CMD) [2,3,4,5] and quantum (QMD) [6,7] molecular dynamics have been used for investigation of uranium thermophysical properties. Although first-principles calculations give more accurate results, their immense computational cost makes the CMD approach more preferable for a study of phase diagrams of the actinides.…”