High-Resolution $^{125}$Te NMR of Novel Thermoelectric Materials

Several novel Te-based thermoelectric materials with extraordinary figure of merit \textit{ZT} $\ge $1.4 have been studied by high-resolution 25 kHz magic angle spinning $^{125}$Te nuclear magnetic resonance (NMR) in order to investigate variations in composition on the nano-scale. A 20-fold wider $^{125}$Te NMR signal of both AgSbGe$_{4}$Te$_{6}$ and AgSbGe$_{5.67}$Te$_{7.67}$ ($\sim $90 kHz) compared to that of PbTe (4.5 kHz) indicates a variation of shifts due to local composition fluctuations. The similar total shift of the main peak in Ag$_{0.53}$Pb$_{18}$Sb$_{1.2}$Te$_{20}$ (-1790 ppm) and PbTe (-1750 ppm) and similarly long T$_{2}$ relaxation time show that the majority of Te atoms in both materials has a similar environment. A second peak in Ag$_{0.53}$Pb$_{18}$Sb$_{1.2}$Te$_{20}$ at -1600 ppm shows the presence of a second type of Te site, accounting for $\sim $1/3 of all Te. These are apparently located in {\{}Ag,Sb{\}}-rich inclusions, as indicated by a much shorter T$_{2}$, which can be due to the effect of quadrupolar relaxation of $^{121}$Sb or $^{123}$Sb (spin 5/2 or 7/2, respectively) on $^{125}$Te. Our data confirm suggestions made by Hsu\textit{ et al., }Science (2004) and by Chen \textit{et al.}, Appl. Phys. Lett. (2005) about the presence of nano-scale inclusions in Ag$_{0.53}$Pb$_{18}$Sb$_{1.2}$Te$_{20}$, which result in low lattice thermal conductivity and high \textit{ZT}.