Employing Interfaces with Metavalently Bonded Materials for Phonon Scattering and Control of the Thermal Conductivity in TAGS-x Thermoelectric Materials

The thermoelectric compound (GeTe)(x)(AgSbTe2)(1-)(x), in short (TAGS-x), is investigated with a focus on two stoichiometries, i.e., TAGS-50 and TAGS-85. TAGS-85 is currently one of the most studied thermoelectric materials with great potential for thermoelectric applications. Yet, surprisingly, the lowest thermal conductivity is measured for TAGS-50, instead of TAGS-85. To explain this unexpected observation, atom probe tomography (APT) measurements are conducted on both samples, revealing clusters of various compositions and sizes. The most important role is attributed to Ag2Te nanoprecipitates (NPs) found in TAGS-50. In contrast to the Ag2Te NPs, the matrix reveals an unconventional bond breaking mechanism. More specifically, a high probability of multiple events (PME) of approximate to 60% is observed for the matrix by APT. Surprisingly, the PME value decreases abruptly to approximate to 20-30% for the Ag2Te NPs. These differences can be attributed to differences in chemical bonding. The precipitates' PME value is indicative of normal bonding, i.e., covalent bonding with normal optical modes, while materials with this unconventional bond breaking found in the matrix are characterized by metavalent bonding. This implies that the interface between the metavalently bonded matrix and covalently bonded Ag2Te NP is partly responsible for the reduced thermal conductivity in TAGS-50.