Thermoelectric properties of Ag<sub>2</sub>S superionic conductor with intrinsically low lattice thermal conductivity

Abstract

Recently, Ag<sub>2</sub>S superionic conductor has attracted great attention due to its metal-like ductility and deformability. In this work, the single phase Ag<sub>2</sub>S compound is fabricated by the melting-annealing method. The crystal structure, ionic conduction, and electrical and thermal transports in a temperature range of 300－600 K are systematically investigated. The monoclinic-cubic crystal structure transition occurs around 455 K for Ag<sub>2</sub>S. Significant reduction in the specific heat at constant volume below the Dulong-Petit limit is observed for Ag<sub>2</sub>S after the monoclinic-cubic phase transition, which is attributed to the liquid-like Ag ions existing inside the sulfur framework. Ag<sub>2</sub>S shows typical semiconducting-like electrical transport behavior in the whole measured temperature range. Around 455 K, the ionic conductivity, carrier concentration, carrier mobility, electrical conductivity, and Seebeck coefficient each show an abrupt change. The calculated ionic activation based on the ionic conductivity is 0.076 eV for the body centered cubic Ag<sub>2</sub>S. The calculated band gap based on the electrical conductivity decreases from 1.1 eV for the monoclinic Ag<sub>2</sub>S to 0.42 eV for the body centered cubic Ag<sub>2</sub>S. The abrupt increase of electrical conductivity after the monoclinic-cubic phase transition leads to a maximum power factor around 5 μW·cm<sup>–1</sup>·K<sup>–2</sup> at 550 K. In the whole measured temperature range, Ag<sub>2</sub>S demonstrates an intrinsically low lattice thermal conductivity (below 0.6 W·m<sup>–1</sup>·K<sup>–1</sup>). The calculated phonon dispersion indicates that the weak chemical bonding between Ag and S is responsible for the low lattice thermal conductivity observed in the monoclinic Ag<sub>2</sub>S. Likewise, the presence of liquid-like Ag ions with low ionic activation energy is responsible for the low lattice thermal conductivity for the cubic Ag<sub>2</sub>S. Finally, the Ag<sub>2</sub>S shows the maximum thermoelectric figure of merit of 0.55 at 580 K, which is comparable to the thermoelectric figure of merit reported before in most of Ag-based thermoelectric superionic conductors.