High-temperature phonon stabilization of $\gamma$-uranium from relativistic first-principles theory

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 $\gamma$-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.

Figure 1

Phonon dispersions for $\gamma$-U at 500 K. SCAILD coupled with FPLMTO without spin-orbit interaction (black line solid squares), with SO (black solid line), and with vasp-PAW (red line solid circles).

Figure 2

Phonon dispersion for $\gamma$-U at 1113 K. SCAILD coupled with FPLMTO (SO + OP).

Figure 3

Phonon density of states for $\gamma$-U at 1113 K, calculated (solid line) and inelastic neutron-scattering data taken from Ref. 24.