Structure Change and Rattling Dynamics in Cu₁₂Sb₄S₁₃ Tetrahedrite: an NMR Study

We present a ⁶³Cu and ⁶⁵Cu NMR study of Cu₁₂Sb₄S₁₃, the basis for tetrahedrite thermoelectric materials. In addition to electronic changes observed at the T_c = 88 K metal-insulator transition, we find that locally there are significant structural changes occurring as the temperature extends above T_c, which we associate with Cu atom displacements away from symmetry positions. Spin-lattice relaxation rates (1/ T₁) are dominated by a quadrupolar process indicating anharmonic vibrational dynamics both above and below T_c. We used a quasiharmonic approximation for localized anharmonic oscillators to analyze the impact of Cu rattling. The results demonstrate that Cu-atom rattling dynamics extends unimpeded in the distorted structural configuration below T_c and provide a direct measure of the anharmonic potential well.