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.