Phonon‐Driven Tetrahedral Tilts Enable Ultralow Bulk Thermal Expansion and Interstitial Oxide‐Ion Migration in Phenacite Solid Electrolytes

ABSTRACT Oxide‐ion solid electrolytes are enabling components for high‐temperature electrochemical technologies, including batteries, fuel cells, membrane reactors, sensors, and electrolyzers. The long‐standing challenge is the thermal‐expansion‐coefficients (TECs) mismatch between electrolytes and adjacent device components over broad operating temperature windows. Here, we demonstrate a design paradigm of phonon‐driven negative‐thermal‐expansion (NTE) framework can be leveraged not only to suppress thermal expansion but also to activate interstitial oxide‐ion migration through collective tetrahedral tilts. As a proof of concept, we report the first demonstration of interstitial oxide‐ion conduction in La‐doped flexible phenacite‐type NTE Zn 2 GeO 4 featuring interconnected 4‐ and 6‐membered ring. The resulting material shows ultralow bulk TECs (α V = 7.232 × 10 −6 K −1 , 298–1273 K) and near‐zero macroscopic linear thermal expansion (α L = 0.307 × 10 −6 K −1 , 298–700 K), corresponding to the lowest reported bulk thermal‐expansion coefficient among oxide‐ion‐conducting solids. Interstitial‐oxygen species are accommodated within 4‐membered rings predominantly coordinated by GeO 4 tetrahedra, and long‐range migration proceeds from 4‐ to 6‐membered rings, coupling the correlated interstitial‐oxygen disorder with the phonon‐driven NTE behavior involving collective tilts of the intrinsic flexible ZnO 4 and GeO 4 tetrahedra. This work pioneers the integration of NTE behavior with oxide‐ion transport, offering a promising strategy to mitigate TEC mismatch in solid‐state ionic devices.