Thermal, electronic and thermoelectric properties of TiNiSn and TiCoSb based quaternary half Heusler alloys from ab-initio calculations
The theoretical analysis about the effect of multi-site substitution based on 18 valence electron count (VEC) rule on electrical, thermal and thermoelectric properties of two well-known half Heusler (HH) alloys, TiNiSn and TiCoSb are reported. The structural optimization and electronic structure calculations were made for 54 quaternary substituted HH alloys using projected augmented wave method and the thermoelectric properties were investigated using the full potential linearized augmented plane wave method, with the semi-classical Boltzmann transport theory, under the constant relaxation time approximation. From the band structure calculations, we show that the multi-site substitution of nonisoelectronic atoms with 18 VEC also one can design new semiconducting HH alloys and tune the bandgap, electronic structure and hence the transport properties. The substituted systems create inhomogeneity in the lattice and more phonon scattering centre due to the presence of large mass difference of the substituted atoms with that in the host lattice and hence lower the thermal conductivity. As a result, it is expected to improve thermoelectric figure of merit (ZT). The selected quaternary systems such as Sc0.5Nb0.5NiSn shows the maximum ZT values of 0.5 at 700K. The calculated phonon spectra and heat capacity show that the substitution at Ti and Sn site in TiNiSn and Ti and Sb site in TiCoSb introduce more optical-acoustic band mixing, which results more acoustic-optical phonon-phonon scattering that is expected to lower thermal conductivity. From the calculated power factor as a function of chemical potential (μ) we have identified optimal p-type or n-type doping levels to achieve maximum ZT and this will provide the guidance to the experimentalists to identify appropriate compositions to synthesize higher efficiency thermoelectric (TE) materials. Based on the calculated results we show that there is lot of potential for designing HH alloys with high ZT by nonisoelectronic multinary substitution.