Electronic inhomogeneity and Ag:Sb imbalance of Ag_(1-y)Pb_(18)Sb_(1+z)Te_(20) high-performance thermoelectrics elucidated by ~(125)Te and ~(207)Pb NMR

摘要

Using magic-angle spinning ~(125)Te and ~(207)Pb NMR, we have discovered the presence of two phases of approximately tenfold different free-electron concentration, n, in high-performance thermoelectrics Ag_(1-y)Pb_(18)Sb_(1+z)Te_(20) ("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 n ≈ 2 × 10~(19) cm~(-3) and 0.2 × 10~(19) cm~(-3), respectively, determined from the spin-lattice relaxation times. ~(125)Te 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 AgSbTe_2), 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.