Quasi-harmonic and Anharmonic Entropies in Transition Metals
Density functional theory (DFT) employing the quasi-harmonic approximation (QHA) is a robust method for evaluating thermal properties of solids. In the case of transition metals however, the method yields high values of the thermal pressure when compared to experimental data or with more direct methods like quantum-molecular dynamics (QMD) simulations. Surprisingly, there has not been to date, a systematic study aimed at understanding the reasons for these large discrepancies, particularly at low temperature, i.e. below the Debye temperature of the solid. Using Tantalum as a test model for which a lot of experimental data exist, thermal properties were evaluated employing direct molecular dynamics (MD) simulations and compared to QHA predictions. The atomic interactions were modeled using an embedded-atom method (EAM) potential for Ta fit to a large data set of ab-initio data. The free energy and entropies are computed as a function of temperature with MD employing the adiabatic switching formalism at zero and 50 GPa. Compared with the quasi-harmonic entropy computed within QHA, the anharmonic entropy is large even at moderately low temperatures, suggesting that phonon frequencies are temperature-dependent at low temperatures but this dependence is significantly less important at high temperatures, where volume-dependence dominates.
K. C., Bimal, "Quasi-harmonic and Anharmonic Entropies in Transition Metals" (2019). ETD Collection for University of Texas, El Paso. AAI27671354.