Jahn-Teller Driven Electronic Instability in Thermoelectric Tetrahedrite

Tetrahedrite, Cu12Sb4S13, is an abundant mineral with excellent thermoelectric properties owing to its low thermal conductivity. The electronic and structural origin of the intriguing physical properties of tetrahedrite, including its metal-to-semiconductor transition (MST), remains largely unknown. This work presents the first determination of the low-temperature structure of tetrahedrite that accounts for its unique properties. Contrary to prior conjectures, the results show that the trigonal-planar copper cations remain in planar coordination below the MST. The atomic displacement parameters of the trigonal-planar copper cations, which have been linked to low thermal conductivity, increase by 200% above the MST. The phase transition is a consequence of the orbital degeneracy of the highest occupied 3d cluster orbitals of the copper clusters found in the cubic phase. This study reveals that a Jahn-Teller electronic instability leads to the formation of "molecular-like" Cu-5(7+) clusters and suppresses copper rattling vibrations due to the strengthening of direct copper-copper interactions. First principles calculations demonstrate that the structural phase transition opens a small band gap in the electronic density of states and eliminates the unstable phonon modes. These results provide insights on the interplay between phonon transport, electronic properties, and crystal structure in mixed-valence compounds.