Thermal Anharmonicity Of Transition Metals: A Case Study Of Tantalum

Density functional theory (DFT) is commonly used for evaluating the thermal propertiesand equation of state of solids, usually within the quasi-harmonic approximation (QHA). In transition metals, thermodynamic quantities such as the thermal pressure and Gruneisen parameter are poorly reproduced compared with experimental values or with direct computational methods like quantum-molecular dynamics (QMD) simulations. Surprisingly, there has not been to date, a systematic study aimed at understanding the reasons for these discrepancies. Using Tantalum as a case study, we have evaluated the temperature dependence of the phonon frequencies employing Harmonic Ensemble Lattice Dynamics (HELD) combined with classical and quantum molecular dynamics. Our results show that phonon frequencies in tantalum are highly temperature-dependent and effective anharmonic effects, lead to significant softening in the transverse acoustic modes, which in turn leads to significant lowering in the calculated values of the Gruneisen parameter, even at room temperature. Our calculations align well with experimental observations and previous studies, confirming the importance of incorporating temperature effects for accurately predicting the thermal properties and equation of state of Tantalum.