Crystal Structure and Atomic Vacancy Optimized Thermoelectric Properties in Gadolinium Selenides

Thermoelectric materials enable the mutual energy conversion of waste heat and electricity, critical to relieve global energy crisis. Hightemperature thermoelectric materials are special species due to their high-temperature stability and noticeable energy conversion efficiency. Here, we report a systematic investigation on high-temperature thermoelectric gadolinium selenides, cubic Gd3-xSe4 (x = 0.16, 0.21 and 0.25) and orthorhombic Gd2Se3-y (y = 0.02, 0.06 and 0.08). High energy synchrotron x-ray diffraction and total scattering are used to investigate the crystallographic and local structures. The atomic-scale cluster of Gd vacancy in cubic Gd2.84Se4 is observed by employing the reverse Monte Carlo simulation. For cubic Gd3-xSe4, its carrier concentration is tuned and multiple conduction bands are incorporated by adjusting Gd vacancy. Experimentally, the gradual increase in effective masses is evidently observed in cubic Gd3-xSe4, which is consistent with the density functional theory (DFT) calculation. A reasonable peak zT value of 0.27 is achieved at 850 K for Gd2.84Se4. On the other hand, adjusting Se vacancy enables the optimization of electron concentration for orthorhombic Gd2Se3-y phase. Its low deformation potential (Ξ = 12eV) with single conduction band gives rise to enhanced electron mobility and higher peak zT value of 0.54 at 850 K for Gd2Se2.98. In addition, a higher zT of 1.2 at1200 K is reasonably predicted for Gd2Se2.98 by using quality factor analysis. This work not just presents a systematic crystallographic investigation of gadolinium selenides, but also provides a deep insight into the charge transport and phonon scattering mechanisms. This study facilitates the exploration of more high-temperature thermoelectric materials. Ministry of Education (MOE) Accepted version The authors would like to acknowledge EFRC Solid-State-Thermal Energy Conversion Center (S3TEC), Grant DE-SC000XXXX. This work is also supported by Japan Society for the Promotion of Science (JSPS) KAKENHI, Grant JP 19F19057. L. H. acknowledge the International Research Fellow of JSPS. Q. F. Y. acknowledge the Chinese Scholarship Council (CSC) for the scholarship in Tokyo Institute of Technology. A. Q. Yan acknowledges Singapore MOE Tier 2 under Grant MOE2018-T2-1- 010. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, under Contract No. DE-AC02-06CH11357.