Thermophysical properties of titanium and vanadium nitrides: Thermodynamically self-consistent approach coupled with density functional theory

In this study, density functional theory (DFT) with a thermodynamically self-consistent (TSC) method are used to predict the thermophysical properties of metallic compounds. The TSC method used in this work is, in summary, an extension of the classical quasi-harmonic approximation satisfying the Maxwell relations and thus ensuring thermodynamic consistency. Electronic band structure and density of state (DOS) of titanium nitride (TiN) and vanadium nitride (VN) are calculated by generalized gradient approximation (GGA) and local density approximation (LDA) to reveal their metallic character. The electron contribution to the heat capacity and thermal expansion is deduced from the DOS and Fermi-Dirac distribution at temperatures above the ground state. The vibrational contribution to thermophysical properties at high temperatures is calculated using the TSC method as an alternative for the existing quasi-harmonic approximations. In addition, elastic and mechanical properties of TiN and VN are calculated. These results show that VN is more anisotropic than TiN, which is manifested in the calculated results. It has been shown that this anisotropic character is rooted in the electronic structure and charge distribution in VN. Thus, the electron localization function (ELF) is calculated to highlight the way in which electrons are localized and the nature of the chemical bonding between the atoms. The significance of the anharmonicity contribution, due to phonon-phonon interaction, for the thermal properties is discussed. Predicted heat capacities and thermal expansions by the TSC method accord well with experimental data. Overall, it has been shown that the TSC method, together with consideration of the electronic structure, is a powerful tool to predict the thermophysical properties of non-magnetic metallic systems, with a high accuracy.