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 Tc = 88 K metal-insulator transition, we find that locally there are significant structural changes occurring as the temperature extends above Tc, 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 Tc. 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 Tc and provide a direct measure of the anharmonic potential well.