Synergistic Suppression of Ionic Conduction and Lattice Thermal Conductivity Through Trigonally Coordinated [CuI 3 ] 2− Units and Zero‐Dimensionality in Cs 3 Cu 2 I 5

Semiconductors with ultralow lattice thermal conductivity (κ L ) are promising for thermoelectric and thermal barrier applications. Superionic materials often exhibit ultralow κ L because of their weak chemical bonds and long‐range ion migration, but their practical use is limited by chemical instability under a temperature gradient or an electrical field. Herein, we demonstrate that the synergy of the trigonally coordinated [CuI 3 ] 2− units and the zero‐dimensional (0D) structure of Cs 3 Cu 2 I 5 provides a platform for simultaneously achieving ultralow κ L and negligible ionic migration. Electron diffraction analysis reveals that the weakly bonded, confined trigonally coordinated [CuI 3 ] 2− units promote Cu + rattling, evidenced by a large anisotropic atomic displacement parameter of 0.105 Å 2 at 96 K, indicating strong anharmonicity. Meanwhile, the 0D framework effectively inhibits Cu + migration, giving Cs 3 Cu 2 I 5 an ultralow κ L of 0.15‐0.14 W/mK and negligible ionic conductivity over 300–473 K. Theoretical calculations further unveil that the ultralow lattice thermal conductivity originates from the combined effects of low sound velocity and strong anharmonic phonon scattering, leading to concurrently suppressed particle‐like and wave‐like thermal transport. These findings establish that weakly bonded units confined within a 0D framework constitute an effective design strategy for simultaneously achieving ultralow κ L and suppressed ionic conduction.