First-principles investigation of electronic, mechanical and thermodynamic properties of L1₂ ordered Co₃(M, W) (M = Al, Ge, Ga) phases

Studies were carried out on the equilibrium structural, temperature-dependent mechanical and thermodynamic properties of the Co ₃(M, W) (M = Al, Ge, Ga) phases in terms of first-principles calculations. The results of the ground-state elastic constants revealed that Co₃(M, W) phases are mechanically stable and possess intrinsic ductility. It was found that the elastic heat-resistant properties of Co ₃(Ge, W) phase are inferior to those of Co₃(Al, W) and Co₃(Ga, W). Analyzing the charge density difference provides the explanation that the sharp decrease in mechanical properties is mainly due to the weakening of Co-Ge bonding at elevated temperatures for Co₃(Ge, W). The elastic anisotropy as a function of temperature is discussed using a universal index. It is observed that Co₃(M, W) phases show a high degree of elastic anisotropy. The degree of elastic anisotropy could be significantly decreased by an increase in temperature for Co₃(M, W). The lattice vibration is treated with the quasiharmonic phonon approach, considering both the vibrational and thermal electronic contributions. The thermodynamic properties as a function of temperature are computed without any adjustable parameters, including heat capacity, entropy, enthalpy and thermal expansion coefficient.