Structure and thermoelectric performance of Ag<sub><i>y</i></sub>In<sub>3.33–<i>y</i>/3</sub>Se<sub>5</sub> compounds

Abstract

In this study, we find new Ag<sub><i>y</i></sub>In<sub>3.33–<i>y</i>/3</sub>Se<sub>5</sub> compounds in Ag-In-Se system by static diffusion method combined with common X-ray diffraction and backscattering electron analysis. The crystal structure belongs to the trilateral system with the<i> P</i>3<i>m</i>1 space group, which features a two-dimensional layered structure. The unit cell is composed of 9-atom quantum layers arranged in the sequence of Se1-In1-Se2-In2-Se3-Ag/In3-Se4-In4-Se5, and in-between these layers are bonded by the weak van der Waals force. The sintered bulk samples show highly anisotropic transport properties and have an ultra-low lattice thermal conductivity along the direction parallel to sintering pressure about 0.15 W·m<sup>–1</sup>·K<sup>–1</sup> at 873 K. The intrinsically ultra-low lattice thermal conductivity mainly comes from low phonon velocity and the strong coupling between low frequency optical phonon and acoustic phonons. The Ag<sub><i>y</i></sub>In<sub>3.33–<i>y</i>/3</sub>Se<sub>5</sub> compounds behave as an n-type conduction. The electrical conductivity is 4×10<sup>4</sup> S·m<sup>–1</sup> and the Seebeck coefficient is –80 μV·K<sup>–1</sup> at room temperature. Therefore, Ag<sub><i>y</i></sub>In<sub>3.33–<i>y</i>/3</sub>Se<sub>5</sub> compounds show high electrical transport properties in a wide temperature range, and the power factor is around 5 μW·cm<sup>–1</sup>·K<sup>–2</sup> in a range of 450–800 K. Owing to the ultra-low lattice thermal conductivity along the direction parallel to sintering pressure, Ag<sub>0.407</sub>In<sub>3.198</sub>Se<sub>5</sub> reaches a maximum <i>ZT</i> of 1.01 at 873 K and an average <i>ZT</i> of 0.45 at 300–850 K. The discovery of Ag<sub><i>y</i></sub>In<sub>3.33–<i>y</i>/3</sub>Se<sub>5</sub> expands the n-type copper based chalcogenide and lays an important foundation for the application of copper based chalcogenide.