Rattling And Band-Filling Effects In Substituted Tetrahedrites: An Nmr Study

We present Cu NMR measurements of Ni-, Zn-, Mn-, and Te-substituted tetrahedrites, which show exceptionally low thermal conductivity and high thermoelectric efficiency. The collective NMR shift behavior demonstrates a common electronic behavior for substituted materials distinct from that of the insulating phase of unsubstituted tetrahedrite, Cu12Sb4S13. The results establish the importance of native defects in reducing the residual carrier densities in partially compensated compositions. In addition, the quadrupolar NMR T-1(-1) relaxation rate is shown to provide an excellent measure of the rattling-type anharmonic vibrational behavior, with the results indicating a particularly large rattling effect in a Zn-Ni co-doped sample, which also exhibits very low thermal conductivity, demonstrating the importance of these vibrational modes in controlling the scattering of phonons. The results further indicate that the rattling behavior is not simply tied to the space provided by the bonding configuration of the trigonal Cu ions. We also address the paramagnetic behavior induced by substituted ions and the development of strongly magnetic behavior with Ni-2 substitution as compared to lower substitution levels.