Mobile copper ions as heat carriers in polymorphous copper sulfide superionic conductors

Liquid-like mobile Cu+ ions are generally considered as phonon scatters in copper chalcogenide superionic conductors, but this is recently disproved by a neutron spectroscopy study on lattice dynamics of Cu2Se compound [Voneshen et al., Phys. Rev. Lett. 17, 118 (2017)]. In this work, we provide a different perspective by investigating the thermal transport of three transformable polymorphs of Cu2S compounds with varying Cu+ content. We show that the disordered and mobile Cu+ ions are not the primary factor for suppressing the heat transport. A notable dependence of thermal conductivity on Cu+ content is observed. By correlating the electrically deducted thermal conductivity with the ion motion behavior for beta-Cu2S superionic phase, we reveal that these fast ionic species Cu+ are heat carriers instead, which make an appreciable contribution to thermal conduction. Published by AIP Publishing.