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

Abstract

Studies were carried out on the equilibrium structural, temperature-dependent mechanical and thermodynamic properties of the Co3(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 Co3(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.