Structural transformation and thermoelectric performance in Ag2Te1−xSex solid solution

Abstract

Both β-Ag2Te and β-Ag2Se are narrow band-gap Ag-based chalcogenide but with very different transport properties. Herein, the phase structure and thermoelectric properties of Ag2Te1-xSex (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) samples were systematically investigated. β-Ag2Te and β-Ag2Se can form a complete solid solution. The Ag2Te1-xSex solid solutions are divided into two crystal structure regions, of which the Ag2Te1-xSex (x = 0, 0.2) samples are the monoclinic phase and the Ag2Te1-xSex (x = 0.4, 0.6, 0.8, 1.0) samples are the orthorhombic phase. Ag2Se alloying in Ag2Te obviously increases the carrier concentration in each phase region, resulting in a better electrical property in the orthorhombic phase region. Besides, alloying Ag2Se in Ag2Te softens the chemical bonding, leading to a much larger Grüneisen parameter in orthorhombic phase. In conjunction with the intensified alloy phonon scattering, Ag2Te0.6Se0.4 obtains the lowest thermal conductivity of 0.59 Wm−1 K−1 at 300 K. As a consequence, the ZT value (400 K) and the average figure of merit ZTavg a value (300–400 K) for orthorhombic phase Ag2Te0.2Se0.8 is 0.7 and 0.55 respectively, which are increased by 130% and 77% compared with the monoclinic Ag2Te0.8Se0.2 sample. This work demonstrates that Ag2Te1-xSex compounds with orthorhombic structure shows greater potential as a high-performance thermoelectric material than that with the monoclinic structure.