NMR Study of Doped Tetrahedrite Thermoelectric Materials.

Tetrahedrite thermoelectric materials with compositions between Cu₁₂Sb₄S₁₃ and Cu₁₀ Zn₂Sb₄S₁₃ were analyzed from room temperature to 4.2 K by ⁶³Cu and ⁶⁵Cu NMR. Cu₁₂Sb₄S₁₃ is known to have a metal-insulator phase transition at T = 88 K, however, structural details of the transformation are not very well known, particularly in the substituted materials of interest for thermoelectric devices. Cu NMR is very sensitive to changes in local structure, and we found the metal-insulator transition still present for Cu₁₁ZnSb₄S₁₃, but absent in the Zn₂ material. However, differences in high-temperature NMR spectra reveal a distinct local structure for Zn₂ vs the undoped material. Also, we found NMR evidence for Cu-ion hopping, which disappears in the Zn₂ material. Spin-lattice relaxation rates (1/T₁) furthermore provide a measure of local anharmonic vibrational motions, through an electric quadrupolar process. Results indicate that the rattling behavior is little changed by Zn substitution up to Zn₂, despite the structural differences.