Electronic inhomogeneity and Ag:Sb imbalance ofAg1−yPb18Sb1+zTe20high-performance thermoelectrics elucidated byT125eandP207bNMR

Abstract

Using magic-angle spinning {sup 125}Te and {sup 207}Pb NMR, we have discovered the presence of two phases of approximately tenfold different free-electron concentration, n, in high-performance thermoelectrics Ag{sub 1?y}Pb{sub 18}Sb{sub 1+z}Te{sub 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 {approx} 2 x 10{sup 19} cm{sup -3} and 0.2 x 10{sup 19} cm{sup -3}, respectively, determined from the spin-lattice relaxation times. {sup 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{sub 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.