Assessment of thermoelectric performance of half-Heusler materials: GW, HSE and DFT methods compared
Peter Kratzer, Physics/University Duisburg-Essen, Duisburg, GermanyMaedeh Zahedifar, Physics/University Duisburg-Essen, Duisburg, Germany
Three ab initio approaches to the band structure of ANiSn and ACoSb half-Heusler compounds (A= Ti, Zr,Hf) are compared and their consequences for thermoelectric properties explored. The GW calculations confirm the trend of a smaller band gap (0.75 to 1.05eV) in ANiSn compared to the ACoSb compounds (1.13 to 1.44 eV) already expected from the DFT calculations. While in ANiSn materials the GW band gap is 20% to 50% larger than in HSE, the fundamental gap of ACoSb materials is smaller in GW compared to HSE. Using the calculated band structures and scattering rates taking into account the band effective masses, the Seebeck coefficients, thermoelectric power factors and figures of merit ZT are predicted for all six half-Heusler compounds. Comparable performance is predicted for the n-type ANiSn materials, whereas clear differences are found for the p-type ACoSb materials. Using the most reliable GW electronic structure, ZrCoSb is predicted to be the most efficient material with a power factor of up to 0.07 W/(K² m) at a temperature of 600K. We find strong variations among the different ab initio methods not only in the prediction of the maximum power factor and ZT value of a given material, but also in comparing different p-type thermoelectric materials. We conclude that the most elaborate, but also most costly GW method is required to perform a reliable computational search for the optimum material.