Exploring the thermal and ionic transport of Cu+ conducting argyrodite Cu7PSe6

Understanding the origin of low thermal conductivities in ionic conductors is essential for improving their thermoelectric efficiency, although accompanying high ionic conduction may present challenges for maintaining thermoelectric device integrity. This study investigates the thermal and ionic transport in the Cu+ ionic conductor Cu7PSe6, aiming to elucidate the fundamental origins of both transport phenomena, and their correlation with the structural and dynamic properties. Through a comprehensive approach including various characterization techniques and computational analyses, it is demonstrated that the low thermal conductivity in Cu7PSe6 arises from structural complexity, variations in bond strengths, and high lattice anharmonicity leading to pronounced diffuson transport of heat and fast ionic conduction. It is found that upon increasing the temperature, the ionic conductivity increases by orders of magnitude in Cu7PSe6 whereas the thermal conductivity remains nearly constant, revealing no direct correlation between ionic and thermal transport. In thermoelectric applications, this absence of direct influence suggests the feasibility of innovative design strategies to enhance stability by diminishing ionic conduction, while at the same time maintaining low thermal conductivity, thereby linking the domains of solid-state ionics and thermoelectrics. Thus, this study attempts to clarify the fundamental principles governing thermal and ionic transport in Cu+ superionic conductors and finds that similar fundamental transport connections as recently found in the Ag+ arygrodites are at play.