FIG. 1. Schematic diagram for designing high performance TE materials. These strategies and concepts are used to maximize the thermoelectric figure of merit ZT value. Here B refer as a dimensionless material parameter

The alloys with 8/18/24 valence electron count (VEC) are promising candidates for efficient energy conversion and refrigeration applications at low as well as high temperatures. Recently Fe− based Heusler alloys attracted researchers due to their compelling electronic band structure i.e flat band along one direction of the Brillouin zone and highly dispersive bands along the other directions.Here we focus on the thermoelectric (TE) transport properties of isovalent/aliovalent substituted Fe2TiSn systems those may be potential TE materials. The multinary substitution has been done in such a way that it preserves the 24 VEC and hence the semiconducting nature. The calculated total energies with VASP−PAW potential within density functional theory with PBE−GGA functional were used to determine the ground state properties such as equilibrium lattice parameters, bulk modulus etc. We have also investigated the structural, electronic, lattice dynamic and TE transport properties by using PBE−GGA and TB−mBJ exchange−correlation functional. The full potential linearized augmented plane wave method as implemented in WIEN2k code was used to investigate electronic structure and TE transport properties with the PBE−GGA and TB−mBJ exchange potentials and Boltzmann transport theory. The calculated single crystal elastic constants, phonon dispersion and phonon density of states confirm that these systems are mechanically and dynamically stable. The TE transport properties is calculated by including the lattice part of thermal conductivity (κL) obtained from two methods one from the calculated elastic properties calculation (κ elastic L ) and the other from phonon dispersion curve (κ phonon L ). The strong phonon−phonon scattering by large mass difference/strain fluctuation of isovalent/aliovalent substitution at Ti/Sn sites of Fe2TiSn reduces the lattice thermal conductivity which results in high ZT value of 0.81 at 900 K for Fe2Sc0.25Ti0.5Ta0.25Al0.5Bi0.5. The comparative analysis of TE transport properties using the band structures calculated with the PBE−GGA and TB−mBJ functional shows that the ZT value obtained from TB−mBJ scheme is found to be significantly higher than that based on PBE−GGA. The calculated relatively low lattice thermal conductivity and high ZT values suggest that isovalent/aliovalent substituted Fe2TiSn are promising candidates for medium to high temperature waste heat recovery.