Electronic inhomogeneity and Ag:Sb imbalance of Ag1−yPb18Sb1+zTe20 high-performance thermoelectrics elucidated by 125Te and 207Pb NMR

Authors

E. M. Levin, Ames Laboratory DOEFollow
B. A. Cook, Ames Laboratory DOE
K. Ahn, Northwestern University
M. G. Kanatzidis, Northwestern University
Ames Laboratory DOE, Ames Laboratory DOE

Date of this Version

2009

Citation

Physical Review B 80, 115211 2009; DOI: 10.1103/PhysRevB.80.115211

Abstract

Using magic-angle spinning ¹²⁵Te and ²⁰⁷Pb NMR, we have discovered the presence of two phases of approximately tenfold different free-electron concentration, n, in high-performance thermoelectrics Ag_1−yPb₁₈Sb_1+zTe₂₀ (“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 × 10¹⁹ cm⁻³ and 0.2 × 10¹⁹ cm⁻³, respectively, determined from the spin-lattice relaxation times. ¹²⁵Te 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.