Band structure, thermoelectric properties, effective mass and electronic fitness function of two newly discovered 18 valence electrons stable half-Heusler TaX(X=Co,Ir)Sn semiconductors: A density functional theory approach

We performed detailed first-principles Density Functional Theory (DFT) Calculations of electronic band structure and predict the effective mass as well as the electronic fitness function S-2 sigma/tau (N/V)(-2/3) in the valence and conduction band edges of two Tantalum-Tin based Half-Heusler Alloys. Implementation of a generalized gradient approximation based DFT coupled with Boltzmann's transport theory allows us to determine the power factor at an appropriate chemical potential (and consequently, an optimum carrier concentration) at which other transport properties were determined. In the pure compounds, the electronic fatbands, both in the valence and conduction bands revealed the great influence of the 3-d orbitals of Cobalt atoms in lowering of the energy band gap of compound, while the 5-d Iridium orbitals contribute very little to energy gap lowering. The Seebeck coefficient is found to be hole dependent as well as having a strong dependence on temperature as it decreases with increasing electron carrier concentration in both compounds. At some high temperatures, the Seebeck coefficient developed high shoulders in the Ir-based compound at high hole concentrations close to 10(22) cm(-3). We found the Co-based compound to be a better thermoelectric, since the Seebeck coefficient shows a heavy-doped traits, while the Ir-based compound demonstrates a parabolic-shaped Seebeck coefficient (which is a sign of a lightly-doped thermoelectric). From the effective mass calculations, we found that an increase in carrier concentrations results in lowering of the effective mass in the conduction band.