Investigation of Electronic and Thermoelectric Properties of Bulk and [001] Thin Film Structures of Half-Heusler Compound TiNiSn: A DFT Study

The electronic and thermoelectric properties of bulk and [001] surfaces of half-Heusler compound TiNiSn are investigated consistently by combining the first principle electronic structure calculations and BOLTZRAP transport theory. It is found that semi-metallic behavior in the Fermi level compared with the bulk state of a compound is declined and just at Ti-Sn termination, a gap near the Fermi surface exists. Our calculations have indicated that Ti-Sn termination has a high density near the Fermi surface which is mostly due to the d orbitals of Ni atoms. Since the power factor and figure of merit depend on electrical conductivity, the decisive factor in determining the thermoelectric performance of materials is their thermal conductivity which, according to the calculations we performed, in Ti-Sn termination, with increasing temperature, the lattice thermal conductivity is decreased logarithmically. So we can conclude thin films can be better thermoelectric materials in high temperatures rather than bulk phases from the same compound. The most amount of figure of merit, which we could obtain at about 1000 K is 0.76 and 0.72 for Ti-Sn termination and bulk phase, respectively. Phase diagram calculations were performed for [001] surfaces of TiNiSn compound and stability of all terminations was investigated.