First-principles analysis of the structural, thermodynamic, elastic and thermoelectric properties of LuXCo2Sb2 (X = V, Nb and Ta) double half Heusler alloys

We used the pseudopotential plane wave approach, as implemented in the Quantum Espresso program, to investigate the impact of X atoms (V, Nb, and Ta) on the physical properties of LuXCo2Sb2 double half Heusler alloys. We determined the equilibrium structural parameters, including the crystal lattice parameters, atomic position coordinates, and bulk modulus, with and without including the spin-orbit effects. The predicted singlecrystal elastic constants (Cij) show that the title compounds are mechanically stable with a pronounced elastic anisotropy. The bulk modulus, shear modulus, Young's modulus, Poisson coefficient, Debye temperature, and Vickers hardness coefficient were deduced from Cij via the Voigt-Reuss-Hill approximations. We also determined the variations of some macroscopic physical parameters as functions of temperature and pressure, namely the thermal expansion coefficient, lattice thermal conductivity, heat capacity at constant volume, Debye temperature and entropy. The considered alloys demonstrate special thermal properties under pressure and heat conditions, specifically, their low thermal expansion coefficient and lattice thermal conductivity; their thermal expansion coefficient is lower than 4.5 x 10-5 K-1 at 1000 K, and lattice thermal conductivity don't exceed 1 W.m- 1 K-1 for temperatures higher than 300 K. Furthermore, we investigated the temperature and charge carrier concentration dependencies of some thermoelectric coefficients. The results of this study reveal the potential of the considered compounds for achieving a figure of merit greater than 0.5 at a temperature of 500 K and a doping concentration of 1020 cm-3.