First-Order Metal–Semiconductor Transition Triggered by Rattling Transition in Tetrahedrite Cu₁₂Sb₄S₁₃: Cu-Nuclear Magnetic Resonance Studies

Jahn–Teller Driven Electronic Instability in Thermoelectric Tetrahedrite

“…Two alternative mechanisms have been proposed: it has been suggested that weak bonding interactions of Cu(2) with the lone pairs of two neighboring antimony cations drive the anharmonic rattling, 4a or that chemical pressure squeezes the Cu(2) cation out of planar coordination, with the amplitude of the out‐of‐plane rattling increasing when the S 3 triangle is compressed 4d . Contrary to previous work that inferred that the Cu(2) cations are displaced from the S 3 trigonal plane below the MST,4,21 our results provide conclusive evidence that the Cu(2) cations remain within the S 3 trigonal plane, but shortening and lengthening of Cu(2)S distances occurs (Figure 5b and Supporting Information). Moreover, while above the MST the distance between trigonal–planar Cu(2) and each of the two antimony cations is approximately 3.38 Å (Figure 5c), in the tetragonal phase, there are two short Cu(2)‐Sb (i) and one long Cu(2)Sb (i) distances of approximately 3.3 and 3.5 Å, and two long Cu(2)Sb (ii) and one short Cu(2)Sb (ii) distances of approximately 3.5 and 3.3 Å.…”

“…Above the phase transition, electronic band structure calculations show that cubic Cu 12 Sb 4 S 13 is a metal, with the Fermi level located near the top of the valence band (two holes per formula unit) 1b . Our electronic band structure calculations (Figure 4d) reveal that the cubic‐to‐tetragonal phase transition opens a small gap of ≈0.064 eV at the top of the valence band, and hence results in a marked reduction in the density of states at the Fermi level, as previously inferred from 63 Cu NMR measurements 21. The main contributors to states above the gap are those from S and the Cu 5 7+ cluster, as shown in Figure 4e.…”

Zn-Substitution Effect on Metal-Semiconductor Transition in Tetrahedrite Cu12Sb4S13

“…The NMR spectrum consists of the sharp line and the broad one. A similar spectrum was observed in the metallic state of Zn0 [3,6]. Therefore the sharp line and the broad one correspond to the signals from Cu(1) and Cu(2) site, respectively.…”

“…Therefore the sharp line and the broad one correspond to the signals from Cu(1) and Cu(2) site, respectively. As shown in Figs 2(a) and 2(b), the line profile retains the characteristic line shape due to the second order quadrupole effect down to 30 K. This behavior is unlike that of the Zn0 system, where the line profile drastically changes at T MST [3]. Therefore, we concluded that the local structural transformation at Cu(2) site is suppressed by Zn-substitution.…”

“…Rattling has been believed to suppress the thermal conductivity and enhance the thermoelectric performance. In addition, our NMR measurements showed that MST is accompanied with the local structural transformation at Cu(2) site with a freezing of rattling of Cu(2) atom [3]. We also reported the discontinuous increase of local magnetization, which means the decrease of local density of states (DOS) at Cu(1) site.…”