Structures And Thermoelectric Properties Of Cu₄Sn₇S₁₆-based Semiconductors

Cu₄Sn₇S₁₆ is one of the typical Cu-Sn-S ternary compounds with complex rhombohedral structure and space group R-3m.In the unit cell of Cu₄Sn₇S₁₆,it has been reported that 3/4 of the tetrahedral sites and 1/8 of the octahedral sites are occupied by Cu+ ions,and 7/8 of the octahedral sites are occupied by Sn4+ions.This indicates that there are 1/4 of tetrahedral Cu vacancies unoccupied.In this work we have synthesized ternary Cu₄Sn₇S₁₆ compounds with different Sn and Cu in excess,which allows extra Sn or Cu to occupy the intrinsic Cu vacancies.We have therefore investigated the structures and thermoelectric?TE?properties of ternary Cu₄Sn₇S₁₆ compounds with an excess of Sn and Cu,achieving the following results which are summarized blow:For the ternary Cu₄Sn_7+xS₁₆ compounds with Sn in excess,the excess Sn has a site preference in the Cu vacancies with the oxidation state Sn2+ acting as donors.The occupation of Sn2+ unpins the Fermi level?Fr?,and allows the Frentering into the conduction band.Therefore,the carrier concentration has been enhanced by about two orders of magnitude with Sn content increasing from x=0to x=0.5.The lattice contribution ?L has been reduced with Sn content increasing.As a consequence,the thermoelectric figure of merit?ZT?of the compound Cu₄Sn_7+xS₁₆?x=0.5??ZT=0.41 at 863 K?has a doubled value of stoichiometric Cu₄Sn₇S₁₆?ZT=0.20 at 853 K?.In the ternary Cu₄Sn₇S₁₆ compounds with Cu in excess,the bandgap has reduces from 0.94 eV to 0.85 eV,and the carrier concentration enhances by about two orders of magnitude with Cu content increasing from x=0 to x=0.2 for Cu4+xSn7S16.The lattice contribution ?L reduces with Cu content increasing.As a consequence,the maximum thermoelectric figure of merit?ZT?of the compound Cu4+xSn7S16?x=0.2?is 0.3 at 893 K,about 1.5 times that of stoichiometric Cu₄Sn₇S₁₆?ZT=0.20 at 853 K?.Upon Se substitution for S in Cu₄Sn_7.5S₁₆,we have prepared the compounds Cu₄Sn_7.5S₁₆-xSex,and observed that the highest ZT value is 0.45?x=1.0?,which is about 10% higher than that of Cu₄Sn_7.5S₁₆.