Beyond Rattling: Tetrahedrites as Incipient Ionic Conductors

Materials with ultralow thermal conductivity are crucial to many technological applications, including thermoelectric energy harvesting, thermal barrier coatings, and optoelectronics. Liquid-like mobile ions are effective at disrupting phonon propagation, hence suppressing thermal conduction. However, high ionic mobility leads to the degradation of liquid-like thermoelectric materials under operating conditions due to ion migration and metal deposition at the cathode, hindering their practical application. Here, a new type of behavior, incipient ionic conduction, which leads to ultralow thermal conductivity, while overcoming the issues of degradation inherent in liquid-like materials, is identified. Using neutron spectroscopy and molecular dynamics (MD) simulations, it is demonstrated that in tetrahedrite, an established thermoelectric material with a remarkably low thermal conductivity, copper ions, although mobile above 200 K, are predominantly confined to cages within the crystal structure. Hence the undesirable migration of cations to the cathode can be avoided. These findings unveil a new approach for the design of materials with ultralow thermal conductivity, by exploring systems in which incipient ionic conduction may be present. In tetrahedrite, copper ions are mobile at temperatures of 200 K or above, but confined to cages within the crystal structure. The localized diffusional jumps of the copper ions within a confined octahedral environment are effective at suppressing thermal conduction. This work reveals that tetrahedrite behaves as an incipient ionic conductor.image

Automated summary

A new behavior called "incipient ionic conduction" is discovered, which leads to materials with ultralow thermal conductivity. By studying the well-known thermoelectric material tetrahedrite, it is found that copper ions are mobile at temperatures above 200K but mostly confined to cages within the crystal structure, avoiding undesirable migration. These findings provide new insights for designing materials with ultralow thermal conductivity.