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 = 85K. 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 20K 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 nu(Q)(2) approximate to 18.6MHz and an asymmetry parameter eta(2) approximate to 0.03. Below T-MST, the values of nu(Q)(2) and eta(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 nu(Q) from NMR spectra and that from first-principles calculation, we conclude that the Cu(2) atoms are displaced from the sulfur triangles below TMST. 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. We further discuss the relationship among the MST, structural transformation, and crystal structure below T-MST.