Electronic inhomogeneity and Ag:Sb imbalance of Ag1-y Pb18 Sb1+z Te20 high-performance thermoelectrics elucidated by T 125 e and P 207 b NMR

Using magic-angle spinning T 125 e and P 207 b NMR, we have discovered the presence of two phases of approximately tenfold different free-electron concentration, n, in high-performance thermoelectrics Ag1-y Pb18 Sb1+z Te20 ("LAST-18"), proven by pairs of Knight-shifted NMR peaks and biexponential spin-lattice relaxation. The ratio of the phases is typically 2:1 with n2× 1019 cm-3 and 0.2× 1019 cm-3, respectively, determined from the spin-lattice relaxation times. T 125 e NMR spectra show that both phases contain similar concentrations of Sb. The low- n component is assigned to Ag-rich regions with Ag-Sb pairing (but not AgSbTe2), the dominant high- n component to PbTe:Sb resulting from the excess of Sb relative to Ag. The electronic inhomogeneity observed here must be considered in the search for a better understanding of high-performance thermoelectric materials.