Copper occupation and dynamics in Cu rich tetrahedrite; an NMR study

Thermoelectric materials can generate electricity from heat. Recently tetrahedrite with the chemical structure of Cu₁₂Sb₄S₁₃ emerged as the most promising thermoelectric material for mid-temperature applications. Here we report ⁶³Cu NMR measurements for the Cu-rich phase of Cu_12+xSb₄S₁₃ (x ≈ 2) and compared to Cu₁₂Sb₄S₁₃. Based on NMR, T₁ and T₂ measurements, the results demonstrate Cu-ion hopping below room temperature with an activation energy of ∼150 meV for the Cu-rich phase, consistent with superionic behavior. The NMR results also demonstrate the effects of Cu-ion mobility in the Cu₁₂Sb₄S₁₃ phase, but with a larger activation barrier. We identify a small difference in NMR Knight shift for the metallic phase of Cu₁₂Sb₄S₁₃, compared to the Cu-rich phase, and when compared to DFT calculations the results indicate a mix of hyperfine contributions to the metallic shift.