First-principles investigation of electronic, mechanical and thermodynamic properties of L1_2 ordered Co_3(M, W) (M = Al, Ge, Ga) phases

摘要

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 Co_3(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 Co_3(Al, W) and Co_3(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 Co_3(Ge, W). The elastic anisotropy as a function of temperature is discussed using a universal index. It is observed that Co_3(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 Co_3(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.