Investigation of mechanical, lattice dynamical, electronic and thermoelectric properties of half Heusler chalcogenides: A DFT study

Favorable physical properties of many Half Heusler alloys (HHAs), along with the possibility of tuning their electronic structure by changing the composition, make the HHAs promising candidates for thermoelectric and spintronic applications. In the present work, we explore HHAs based on various chalcogen atoms, with 18 valence electrons (VEC) per unit cell, namely, A(=Sc,Y,La)B(=Co,Rh)C(=S,Se,Te). Systematic density functional theory (DFT) based investigations have been carried out to probe their energetic, mechanical and lattice dynamical properties. Following that, analyses of band structure and Fermi surface reveal high values of valley degeneracy in the states near the Fermi energy in many of these alloys. Therefore, subsequently, thermoelectric properties (TEP) of these above-mentioned alloys have been studied. The dependence of the TEP on the carrier concentration, chemical potential and temperature for the n- and p-type doped systems have been studied. An enhancement of 50%–100% in the values of power factor (S²σ/τ) for YCoC(=S,Se,Te) and LaCoC(=S,Se,Te) alloys for the p-type doping has been observed as compared to TiNiSn. In comparison to the well-known TiNiSn alloy, for most of the probed HHAs, about 10% enhancement has been found in the maximum values of figure of merit calculated using only the electronic contribution ((ZT_e)_max). Further, it is essential to calculate the figure of merit (ZT) for total thermal conductivity (κ = κ_e + κ_l). Hence, to obtain the lattice contribution of the thermal conductivity, we employ an equation which uses the Gru¨neisen parameters. Subsequently, we calculate the ZT values using κ. We find that these values show enhancement when compared with the value obtained for TiNiSn. The favorable electronic structure and propitious values of power factor and figure of merit make this class of systems promising for possible thermoelectric applications.