Incipient Ionic Conductors: Ion‐Constrained Lattices Achieving Superionic‐Like Thermal Conductivity Through Extreme Anharmonicity

Phonon liquid-like thermal conduction in the solid state enables superionic conductors to serve as efficient thermoelectric device candidates. While liquid-like motion of ions effectively suppresses thermal conductivity (κ), their high mobility concurrently triggers material degradation due to undesirable ion migration and consequent metal deposition, making it a challenge to balance low κ and high stability. Here, phonon liquid-like thermal transport is reported alongside restricted long-range ion migration in CsCu2I3 with incipient ionic conduction, using synchrotron X-ray diffraction, inelastic X-ray scattering, and machine-learning potential-based simulations. The Cu ions are revealed to exhibit confined migration between CuI4 tetrahedra at high temperatures, displaying extreme anharmonicity of dominated phonons beyond conventional rattling and comparable to that in superionic conductors. Consequently, a glass-like κ (≈0.3 W m-1 K-1 at 300 K) following the relationship of κ ≈ T 0.17, is achieved along the x-direction, where Cu ion migration is three orders of magnitude lower than in superionic conductors. These results highlight the advantage of incipient ionic conductors in simultaneously maintaining both low κ and high stability, elucidating the thermal transport mechanism via ion migration constraints, and paving an effective pathway toward ultralow thermal conductivity in ionic conductors.