NMR study of local structures and anharmonic phonon behavior in tetrahedrite thermoelectrics.*

We report Cu NMR studies of tetrahedrites, including Cu₁₂Sb₄S₁₃ as well as substituted materials, which have been of considerable interest as thermoelectric materials as well as due to the metal-insulator transition present at low temperatures. NMR lineshapes show a splitting of the Cu(1) site at room temperature, a result which is consistent with a split atomic configuration. The results indicate that this behavior is static in pure materials, rather than a dynamical effect. NMR T₁ relaxation data for Cu₁₂Sb₄S₁₃ also are shown to be dominated by an electric quadrupole mechanism for the entire range 4 K to room temperature, indicating an anomalous vibrational contribution. We have fitted to a model based on quasi-localized rattling-type motion, with fitting parameters giving an approximate measure of the anharmonic potential. Samples with substitution by Zn and Ni show increasing paramagnetic Cu NMR shifts, with the largest (2000 ppm) in Cu₁₂Sb₄S₁₃. We have analyzed the data in terms of changes in the metallic shift contribution, and also compare to ab initio calculations of the NMR shifts.