Rattling of Three-Fold Coordinated Copper in Sulfides: A Blockade for Hole Carrier Delocalization but a Driving Force for Ultralow Thermal Conductivity

Copper-rich sulfides are attracting a wide attention for energy conversion applications due to their environment compatibility, cost effectiveness and earth abundance. Herein, based on a comparative analysis of the structural and transport properties of state-of-the art Cu-rich sulfides, we highlight the role of the cationic coordination types and networks on the electrical and thermal properties. By combining experiments and theory, we show that the ultralow thermal conductivity originates mainly from the very high anisotropic thermal vibration of copper due to its threefold coordination. Density functional theory (DFT) calculations reveal that these Cu atoms are weakly bonded and, thus, give rise to low-energy, Einstein-like vibrational modes that strongly scatter the heat-carrying acoustic phonons. Importantly, we demonstrate that the three-fold coordination of copper causes a hole blockade. This phenomenon hinders the possibility to optimize the carrier concentration and electronic properties through mixed valency Cu⁺/Cu²⁺, differently from tetrahedrite and most other copper-rich chalcogenides, where the main interconnected Cu-S network is built of CuS₄ tetrahedra. The comparison between various copper-rich sulfides demonstrates that seeking for frameworks characterized by the coexistence of tetrahedral and threefold coordinated copper is very attractive for the discovery of efficient thermoelectric copper sulphides. Considering that lattice vibrations and carrier concentration are key factors for engineering and controlling all transport phenomena (electronic, phonon, ionic…) in copper-rich chalcogenides for various types of applications, our findings on the structure/properties relationships can serve as new guidelines for the design of functional materials for the generation of sustainable energy with wide-ranging applications.