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

Abstract

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 (TiFe_1−xNi_xSb, ZrFe_1−xNi_xBi and VFe_1−xNi_xGe) 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.