Spinodal decomposition in (Ti, Zr)CoSb half-Heusler: A nanostructuring route toward high efficiency thermoelectric materials

Half-Heuslers (HH) represent an emerging family of thermoelectric (TE) materials, wherein intrinsic doping enables a wide range of electronic functionalities. In recent years, the solid-state transformation phenomenonon of spinodal decomposition has been actively explored as an effective paradigm to attain bulk nanostructured TE materials via induced phase separation. In the present work, the implication of intrinsic doping and spinodal decomposition on the thermal and electrical transport parameters of nonstoichiometric (Ti, Zr)CoSb HH systems is examined and corroborated with the help of microstructural characteristics. The synthesized HH nanocomposites were found to contain coherent nanoscale heterogeneities along with nanoscale grain, which severely depress the lattice thermal conductivity, while the intrinsic doping due to interstitial Co and defects induced by excess Co off-stoichiometry favorably tunes the electrical transport. A maximum ZT of ∼0.7 at 873 K was obtained for optimized p-type ZrCo_1.03Sb_0.8Sn_0.2 HH nanocomposites, which is among the highest obtained in p-type HH alloys. The present work thus provides a fundamental basis to the understanding of defect engineering and to achieve highly efficient and cost effective HH compositions.