Prediction of improved thermoelectric performance by ordering in double half-Heusler materials

Many cobalt (Co)-based 18-electron half-Heusler compounds usually exhibit good thermoelectric properties. Due to the increasing scarcity of Co, replacing Co with Fe and Ni to form double half-Heusler compounds is an effective strategy to form stable nominal 18-electron ground states and eliminate the need for Co. We investigate here the phase stability of three double half-Heusler systems (TiFe1-xNixSb, ZrFe1-xNixBi and VFe1-xNixGe) through density-functional theory combined with a cluster expansion method. Two stable ground state ordered structures (Ti4Fe2Ni2Sb4 and V4Fe2Ni2Ge4) are identified. Based on the calculations of electronic and phonon structures, we find that the two ordered structures can maintain the excellent electrical properties of pristine half-Heusler compounds but with low thermal conductivity as found experimentally. The p-type (n-type) zT values of Ti4Fe2Ni2Sb4 and V4Fe2Ni2Ge4 are predicted to reach 1.75 (0.64) and 1.33 (0.95), respectively. Our work not only provides promising double half-Heusler candidates for further experimental investigation but also suggests that forming ordered structures instead of solid solution is an efficient method to achieve excellent thermoelectric properties in double half-Heusler systems.