Ultralow lattice thermal conductivity and superior thermoelectric performance in AgAlS2 and AgAlSe2

The search for thermoelectric materials with efficient energy conversion capabilities is crucial for addressing energy crises and environmental issues. In this work, we explore two Ag-based chalcopyrites, AgAlS2 and AgAlSe2, as potential thermoelectrics with outstanding performance using first-principles calculations and Boltzmann transport theory. We compute formation energies and phonon spectra to verify their thermodynamic and dynamical stabilities. In addition, we perform ab initio molecular dynamics simulations to confirm their thermal stability at high temperatures. Using the PBE and HSE06 methodologies, these materials are classified as medium-wide indirect band gap semiconductors. Remarkably, at 700 K, AgAlS2 and AgAlSe2 show low lattice thermal conductivities (kappa L) of 0.53 and 0.37 W m-1 K-1, respectively, in the z direction. The ultralow kappa L observed in the AgAlSe2 compound is attributed to the strong phonon anharmonicity and increased scattering rates resulting from the simultaneous presence of antibonding-induced anharmonic rattling of Ag atoms and low frequency optical phonon modes. Furthermore, at optimal carrier doping, the flat and heavy degenerate valence band edges result in high thermopower, while the light bands at the conduction band minimum lead to high electrical conductivity (sigma) and electronic thermal conductivity (ke). Upon combining the high power factor and low kappa L, the predicted figure of merit (ZT) can reach up to 1.55 (2.18) for p-type AgAlS(Se)2 at 700 K, suggesting that Ag-based chalcopyrites are promising thermoelectrics at medium temperature.