High Pressure and Temperature Elasticity and EOS for Actinide Metals from First-Principles Simulations
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High Pressure and Temperature Elasticity and EOS for Actinide Metals from First-Principles Simulations Christine J. Wu1 and Per Söderlind1 1 Condensed Matter and Materials Division, Physical and Life Science Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94551, USA ABSTRACT Density-functional theory (DFT) simulations are applied to obtain elastic, strength, and EOS properties of actinide metals under extreme conditions. In this presentation, we will show our recent study on temperature effects of the properties of solids of actinide metals. For example of low temperature uranium (U) solids, elastic constants are calculated directly from the DFT total energy for the ground-state phase in a wide pressure range. For higher temperature U solids, we are applying a recent scheme to calculate temperature-dependent phonon dispersions through the self-consistent ab initio lattice dynamics (SCAILD) technique. This scheme is particular important for the higher temperature phases that the elasticity cannot be analogously obtained because of its mechanical instability at lower temperatures. From these SCAILD phonon dispersions we then extract the elastic constants from the slopes approaching the Γ point. In addition, the phonon density of states of U obtained from SCAILD/DFT calculations have been used to parameterize a double Debye model for its ion-thermal free energy. We will discuss the ramification of this new Debye model on our development of multi-phase uranium EOS. INTRODUCTION A fundamental description of any material includes an accurate phase diagram and equation-of-state (EOS). The EOS simply describes how the material responds to changes of thermodynamics conditions, such as energy, pressure, density, and temperature. Sometimes the response includes crystallographic phase transitions and/or dramatic changes of the electronic structure. At equilibrium (ambient) conditions experimental data can often be found in the literature to help guide the modeling but when exploring the phase space outside these conditions one needs to rely on theory. Fortunately, since its conception in the 1960’s, the sophistication of density-functional theory has evolved to a level where actinide metals can now be quite accurately modeled from first-principles [1], even at considerable pressures [2]. However, the full temperature dependence in the free energy is not easily obtained from a DFT approach that instead provides the energy at zero temperature, the so-called ‘cold curve’. The temperature broadening and associated entropy of the electronic structure (no vibrational contribution) is straightforwardly calculated within DFT while the effect of lattice dynamics has to be added separately in most cases. Therefore, it is of interest to try and calculate phonons as well as elastic constants for the material, preferably both as functions of temperature and pressure. In the following we will focus on a prototype actinide metal, namely uranium, and describe some of our efforts in this regard. In particular, we are interest
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