Metal-Semiconductor Transition Concomitant with a Structural Transformation in Tetrahedrite Cu12Sb4S13

The tetrahedrite Cu12Sb4S13 undergoes a metal-semiconductor transition (MST) at T-MST = 85 K, whose mechanism remains elusive. Our Cu 2p X-ray photoemission spectroscopy study revealed the monovalent state of Cu ions occupying the two sites in this compound. This fact excludes the possibilities of previously proposed antiferromagnetic order and Jahn-Teller instability inherent in a divalent Cu system. A synchrotron X-ray diffraction study has revealed that the body-centered cubic cell of Cu12Sb4S13 transforms into a body-centered 2a x 2a x 2c tetragonal supercell below T-MST, where the cell volume per formula unit expands by 0.25%. We have further studied pressure effects on the MST as well as the effects of the substitution of As for Sb. The application of pressure above 1 GPa completely inhibits the MST and leads to a metallic state, suggesting that the low-temperature structure with a larger volume becomes unstable under pressure. The As substitution also reduces the volume and suppresses the MST but the full substitution induces another transition at 124 K.