Theoretical insights into the electronic structures and transport properties of Ag3TaX4 (X = S, Se, Te) compounds for energy applications

This work investigates the optoelectronic and transport properties of novel silver-based ternary chalcogenide Ag3TaX4 (X = S, Se, Te) compounds, mainly in the context of energy applications. The structural, electronic, optical, elastic, and transport properties of Ag3TaX4 (X = S, Se, Te) compounds have been probed using the first-principles calculations based on density functional theory (DFT) as computed in the Quantum Espresso (QE) package. The calculated indirect electronic bandgaps are 1.70 eV, 1.50 eV and 1.24 eV for Ag3TaS4, Ag3TaSe4, and Ag3TaTe4, respectively. The density of states (DOS) analysis shows that, from S to Se to Te, the bandgap narrows due to the downward shift of conduction-band minima (CBM) and upward shift of valence-band maxima (VBM), indicating that heavier chalcogen substitution enhances orbital hybridization. The Projected DOS (PDOS) indicates that the VBM is primarily composed of Ag-4d/X-p hybridized states, while the CBM arises from Ta-5d/X-p hybridization. Further, the optical properties including the real and imaginary parts of the dielectric constants, refractive index, and absorption coefficients have been analyzed to understand the optical response of the silver-based ternary semiconductors. The computed elastic constants indicate that these compounds exhibit ductile behavior. Finally, the transport properties such as Seebeck coefficient, electrical and thermal conductivity, power factor, and figure of merit (ZT) are computed using semi-classical Boltzmann transport theory. The maximum values of Seebeck coefficients are estimated as 1250 μVK−1 for Ag3TaS4 at 550 K, 875 μVK−1 for Ag3TaSe4 at 600 K, and 250 μVK−1 for Ag3TaTe4 at 400 K. The Ag3TaS4 compound shows the maximum ZT value of 1.75 at 550 K, whereas that of 0.45 at 400 K for Ag3TaTe4 and 0.50 at 600 K for Ag3TaSe4 compounds. These findings disclose the potential candidatures of Ag3TaX4 (X = S, Se, Te) for the energy harvesting applications.