Origin of the Distinct Thermoelectric Transport Properties of Chalcopyrite ABTe₂ (A = Cu, Ag; B = Ga, In)

Despite the same crystal structure and homologous constituent elements, the chalcopyrite compounds ABTe₂ (A = Cu, Ag; B = Ga, In) exhibit distinct electronic and thermal transport properties. The aim of this work is to understand the origin of such discrepancy employing experiments and theoretical calculations. The results of Hall coefficient measurements, absorption spectroscopy, and electronic transport studies suggest the deep-level in-gap states induced by the native A-site vacancies play a key role in the observed intrinsic semiconductor to degenerate semiconductor transition and are the origins of the distinct electrical conductivity among ABTe₂ compounds. In addition, the cryogenic heat capacity measurements and calculated phonon dispersion relations show that the acoustic and low-frequency optical modes of AgGaTe₂ and AgInTe₂ are governed by the vibrations of Ag-Te clusters while the counterparts of CuGaTe₂ and CuInTe₂ compounds are dominated by the vibrations of Te atoms, and the coupling between the acoustic and low-frequency optical modes is notably different among ABTe₂ compounds. Specifically, lower avoided-crossing frequencies, lower sound velocity together with stronger Umklapp process yield lower thermal conductivities of AgGaTe₂ and AgInTe₂ than CuGaTe₂ and CuInTe₂. This work provides new insights into the understanding and improvement of electrical and thermal properties toward higher thermoelectric performance of chalcopyrite compounds.