Structural, elastic, electronic, thermal, and phononic properties of yttrium carbide: First-principles calculations

In this paper, structural, electronic, elastic, phononic, and thermal properties of YC in both rock-salt (B1) and CsCl (B2) phases are studied. Calculations have been performed by using QUANTUM-ESPRESSO/PWSCF computational package which is based on density functional theory (DFT) and pseudo-potential method. Local density approximation (LDA) and generalized gradient approximation (GGA) have been used for modeling the exchange and correlation effects. According to the total energy diagram versus unit cell volume, the YC compound in the B1 phase is more stable than that of the B2 phase at ambient pressure. The computed elastic parameters indicated that the B2 phase did not satisfy the mechanical stability conditions at ambient pressure and is considered to be an unstable phase of the YC structure. The analysis of the enthalpy versus pressure diagram predicts that the structural phase transition from the B1 phase to the B2 phase occurs at pressures 51 GPa and 71.5 GPa, for various LDA and GGA approximations, respectively. Based on the phonon dispersion curves at ambient pressure, the B1 phase of the YC compound is dynamically stable, while at high pressure, the YC compound in the B2 phase has shown stability. Using calculated elastic constants, the small amount of Zener anisotropy factor A shows a strongly anisotropic nature of the YC structure. The higher amount of the calculated Poisson's ratio (-0.40) than that of the corresponding critical value (-0.33) together with the positive values of Cauchy pressure within the LDA and GGA approximations present the ductility and metallic behavior of the YC compound in the B1 phase. The band structure and total density of states calculations also verify the metallic behavior of the YC compound in the stable B1 phase. From PDOS calculations, overlapping the 4d orbital of the Y atom and 2p orbital of the C atom near the Fermi level indicates that these orbitals have significant contributions to the bonding between the Y and C atoms. This bonding is substantiated by the corresponding electron density distribution. The thermodynamic calculations indicated that the heat capacity increases regularly and continuously with the increase in temperature T, toward the classical Dulong-Petit value of 6R for the bulk structure of the YC compound with two atoms in the primitive cell. It can also be predicted for the YC compound that the smaller lattice parameter within the LDA approximation compared to the GGA approximation causes stiffness increases, followed by an increase in the Debye temperature. The increasing trend of entropy versus temperature indicates the endothermic behavior of the YC compound.