Thermoelectric properties of low thermal conductivity half Heuslers TiXPb (X = Ni, Pd, Pt): A first principles investigation

Semiconducting half-Heusler alloys are potential candidates for thermoelectric generators operational at high temperatures. In this work, the stability, electronic, and thermoelectric properties of 18 valence electron TiXPb (X$=$Ni, Pd, Pt) compounds are investigated using density functional theory and semi-classical Boltzmann transport theory. The compounds are both thermodynamically and dynamically stable. We find them to be semiconductors with indirect band gaps lying between 0.32−0.64 eV. Our calculations show that from thermoelectric performance perspective electrons exhibit better transport properties than holes. A combination of large power factor and low lattice thermal conductivity results in $zT>1$ in all the materials. Our calculations predict that amongst the three compounds, TiPtPb have a maximum value of $zT$ for both electrons and holes. In this material our calculation yields a maximum $zT$ of 2.22 at 900 K for n-type doping at a doping concentration of 9.46 × $10^{20}$ ${\mathrm{cm}}^{-3}$ and 1.80 at 900 K for p-type doping at a doping concentration of 4.51 × $10^{20}$ ${\mathrm{cm}}^{-3}$.