First-order metal–semiconductor transition triggered by rattling transition in tetrahedrite Cu₁₂Sb₄S₁₃: Cu-nuclear magnetic resonance studies

Tetrahedrite Cu₁₂Sb₄S₁₃ shows a metal–semiconductor transition (MST) at T_MST = 85 K. We have studied the mechanism of the MST by measurements of Cu-NMR. Above T_MST, the Cu-NMR spectrum consists of signals from a tetrahedral Cu(1) 12d site and a trigonally coordinated Cu(2) 12e site. Analyses of the spectra at 95 and 20 K yield NMR and nuclear quadrupole resonance (NQR) parameters above and below T_MST. The Cu(1) signal does not show clear quadrupole splitting, which remains unchanged on cooling across T_MST. On the other hand, the Cu(2) signal at T > T_MST clearly shows the quadrupole splitting characterized by a quadrupole frequency ν_Q(2) ≈ 18.6 MHz and an asymmetry parameter η(2) ≈ 0.03. Below T_MST, the values of ν_Q(2) and η(2) change markedly. Such marked changes in NQR parameters provide evidence of a remarkable change in the local electronic structure around the Cu(2) site. Together with ν_Q from NMR spectra and that from first-principles calculation, we conclude that the Cu(2) atoms are displaced from the sulfur triangles below T_MST. We also found that the NMR Knight shift and the spin–lattice relaxation rate divided by temperature, 1=T₁T, for the Cu(1) signal markedly change below T_MST, which is ascribable to the reduction in the electronic density of states at the Fermi level.