Nanoscale clusters in the thermoelectric AgPb$_{m}$SbTe$_{m+2}$ and Ag$_{n}$Sn$_{m}$Sb$_{n}$Te$_{m+2n}$ materials

The local structure of the AgPb$_{m}$SbTe$_{m+2}$ series of high performance thermoelectric materials has been studied using the atomic pair distribution function (PDF) method. The dimensionless thermoelectric figure of merit, $ZT$, of the $m\sim 18$ composition material was found to reach 1.7 at 700 kelvin, compared to the highest observed $ZT$ of only 0.84 for PbTe at 648 kelvin in n-doped material. This is asurprisingly large enhancement in $ZT$ for the addition of just 10\%per formula-unit of silver and antimony ions. It is clearly of the greatest importance to trace the origin of the $ZT$ enhancement. Three candidate-models were attempted for thestructure of this class of materials using either a one-phase or a two-phase modeling procedure. Combining modeling the PDF with HRTEM data we show that AgPb$_{m}$SbTe$_{m+2}$ contains nanoscale inclusions with composition close to AgPb$_{3}$SbTe$_{5}$ randomly embedded in a PbTe matrix. We extended the local structural PDF study to Ag$_{n}$Sn$_ {m}$Sb$_{n}$Te$_{m+2n}$, preliminary results of which suggest the presence of nanoscale inclusions in this system as well.