Quantum molecular dynamics simulations of uranium at high pressure and temperature

Abstract

Constant-volume quantum molecular dynamics (QMD) simulations of uranium (U) have been carried out over a range of pressures and temperatures that span the experimentally observed solid orthorhombic α-U, body-centered cubic (bcc), and liquid phases, using an ab initio plane-wave pseudopotential method within the generalized gradient approximation of density functional theory.A robust U pseudopotential has been constructed for these simulations that treats the 14 valence and outer-core electrons per atom necessary to calculate accurate structural and thermodynamic properties up to 100 GPa. Its validity has been checked by comparing low-temperature results with experimental data and all-electron full-potential linear-muffin-tin-orbital calculations of several different uranium solid structures. Calculated QMD energies and pressures for the equation of state of uranium in the solid and liquid phases are given, along with results for the Grüneisen parameter and the specific heat. We also present results for the radial distribution function, bondangle distribution function, electronic density of states, and liquid diffusion coefficient, as well as evidence for short-range order in the liquid.

Structure, formation energies and elastic constants of uranium metal investigated by first principles calculations

“…Söderlind and Boettger et al have ever calculated the structural parameters of ␣-U by using the full potential version of the linear muffin-tin orbital (FP-LMTO) method [3,4]. Table 1 shows the comparison of the ground-state properties of ␣-U calculated by first principles calculations and the corresponding values obtained in experiments [3][4][5][22][23][24][25]. The agreement between experiment and the calculated results is considerably impressive.…”

“…bct-U were also investigated in present study. The bct-U is of particular interest because it has been predicted that this phase might be observed at high pressure [22][23][24][25]. Table 3 shows the calculated lattice constants and formation energies of these structures.…”

Combined ab initio and empirical model of the thermal conductivity of uranium, uranium-zirconium, and uranium-molybdenum

“…Thermal expansion effects will decrease the density of materials and therefore decrease phonon thermal conductivity [73]. The influence of thermal conductivity, estimated from the Leibfried-Schlemaan formula for phonon-phonon scattering [74] and the Gruneisen parameter of U [75] (which are discussed in Appendix VIII), is ~13% near 600K. Thus, thermal expansion effects may make our predicted thermal conductivity slightly higher than actual values by ~1% above 600K, but the change is expected to be negligible.…”

“…which leads to the following expression by differentiating with respect to volume at constant temperature: , where ≈ 1 [81,82]. For U, = 1.7 [75] and ∆Ñ Ñ ≈ 2% near 600K [83], thus the phonon thermal conductivity decrease is ~13% for 600K. This leads to the total thermal conductivity to decrease <1%.…”

Quantum molecular dynamics

“…10)] would be that for each ion interacting with the other ions by means of the appropriate electronic potentials depending on the quantum trajectories of the other ions. The terminology quantum molecular dynamics in the current literature refers quantum electronic motion and classical ion motion 4; the present theory treats both electrons and ions quantum mechanically.…”