Link between changes in \textit{ZT} and microstructure in AgSbTe$_{2}$

The best thermoelectric alloys have complex microstructures. For example, the LAST alloys, (AgSbTe$_{2})_{1-x}$(PbTe)$_{x}$, possess \textit{ZT}$\sim $1.5-2 but have a great variety of inclusions with different chemistry at different length scales. How does microstructure affect thermoelectric efficiency? Since the phase diagram of this and most quaternary alloys is poorly known, transport properties have not been systematically connected to microstructure. We are attacking this problem by studying the simple ternary alloy AgSbTe$_{2}$, a component of the LAST system, in order to show how thermoelectric transport changes with a known, controlled microstructure. AgSbTe$_{2}$ forms within the well-studied Ag$_{2}$Te-Sb$_{2}$Te$_{3}$ pseudobinary phase diagram. We have found that Sb-rich AgSbTe$_{2}$ is composed of Sb$_{2}$Te$_{3}$ precipitates embedded in a homogeneous rocksalt Ag$_{16}$Sb$_{30}$Te$_{54}$ matrix. The precipitates are plate-like and crystallographically aligned along their close packed planes parallel to that of the matrix. The size of these Sb$_{2}$Te$_{3}$ plates can be tuned from the nanometer to micron scale. In this work, the formation and growth of precipitates over a wide length scale is linked to changes in thermoelectric properties for the first time. This study is useful for understanding the complexity of LAST, or any bulk thermoelectric where second phase precipitation occurs.