Enhancement and manipulation of the thermoelectric properties of n-type argyrodite Ag8SnSe6 with ultralow thermal conductivity by controlling the carrier concentration through Ta doping

Abstract

N-type argyrodite Ag8SnSe6 shows great potential as a novel thermoelectric material from room to middle temperatures. However, its thermoelectric properties cannot be manipulated because of the difficulty in controlling the carrier concentration by electron doping. In this study, to discover a suitable dopant for controlling the carrier concentration, we used Ta5+, Mo6+, and W6+ as dopants for Sn4+ in n-type Ag8SnSe6. Only Ta-doped Ag8SnSe6 was obtained as a single phase, and the thermoelectric properties relevant to the carrier concentration of n-type Ag8Sn1−xTaxSe6 (0.0 ≤ x ≤ 0.05) were experimentally investigated. The lattice thermal conductivities were ultralow (around 0.5 W m−1 K−1) and showed glass-like behavior independent of the temperature. The carrier concentration monotonically increased with increasing x. The power factor and dimensionless figure of merit ZT were related to the value of x, and they showed dome-like behavior as a function of x. The power factors of Ag8Sn1−xTaxSe6 in the orthorhombic phase at 300 K and superionic phase at 390 K reached ∼5.3 µW cm−1 K−2 for x = 0.0375 and ∼4.6 µW cm−1 K−2 for x = 0.05, respectively. The ZT values of ∼0.28 for x = 0.025 in the orthorhombic phase at 300 K and ∼0.51 for x = 0.0375 in the superionic phase at 390 K were ∼40% and ∼220% higher than those of non-doped Ag8SnSe6. Therefore, Ta doping of Ag8SnSe6 can control the carrier concentration and result in enhancement and manipulation of the thermoelectric properties.