Syntheses of a series of BaCu2−xAgxSe2 (x = 0 – 1.0) selenides and evaluation of their thermoelectric properties

Abstract

The development of new sulfide and selenide-based thermoelectric (TE) materials is critical for the mass production of TE devices due to the higher earth abundance of S/Se than Te. In this work, we have prepared single crystals of Ag-substituted BaCu_2−xAg_xSe₂ (x = 0 – 0.8) by direct fusion of elements at high temperatures. As characterized by single crystal X-ray diffraction (SCXRD) studies, each of the structures (space group: Pnma) contains two transition metal sites (M1 and M2), and the Ag atoms are disordered with Cu atoms at both sites with a slight preference for the M2 site. Phase pure polycrystalline BaCu_2−xAg_xSe₂ (x = 0 – 1.0) samples have also been synthesized, which are semiconductors as confirmed by optical absorption and resistivity studies. All these samples show positive values of the Seebeck coefficient, suggesting the p-type electrical conduction. Our thermal conductivity (k_tot) studies demonstrate a remarkable drop in k_tot values on increasing the Ag-substitution in the BaCu_2−xAg_xSe₂ phases with the lowest k_tot value of 0.33 W m⁻¹ K⁻¹ (673 K) for the x = 1.0 sample. Our studies show the potential of the BaCuAgSe₂ for TE applications with higher zT values if the hole concentration of the phase can be fully optimized.