Stability and Thermoelectric Properties of the Canfieldite Ag8SnS6

Thermoelectric sulfides have recently attracted special attention because of the high availability and environmentally benign character of sulfur. In special cases such as in copper and silver sulfides with high ion mobility and thus high disorder, excellent thermoelectric performance can be achieved. However, stability is a high concern; the ion mobility may lead to device degradation over time. Here, we studied the stability of the canfieldite Ag8SnS6 under different measurement conditions. Without any precautionary measures, formation of Ag and or Ag2S wires was observed during the transport measurements on Ag8SnS6. This problem of lack of stability can be mitigated by restricting the maximum temperature to remain below 685 K. Increasing the Ag content enhances the performance by enlarging the electrical conductivity, but comes at the cost of lower stability. In that case, decreasing the current density was required to avoid this degradation. Such a limitation is practical in small-scale applications such as thin-film devices, but not in large-scale power generation applications.