Isotropic zero thermal expansion in sodalite crystals from 11 to 893 K

Abstract

Isotropic zero thermal expansion (ZTE) materials are essential for applications requiring extreme dimensional stability, yet their operation is typically limited to narrow temperature windows below 400 K. Here we report the incorporation of flexible interstitial groups via fractionally occupied atoms into a closed-framework sodalite structure (Cd4Al6O12SO4, CASO). This material exhibits isotropic ZTE, with a thermal expansion coefficient of 0.21(23) × 10−6 K−1 from 11 K to 893 K. The high-temperature ZTE behaviour originates from the preserved transverse vibrations that drive negative thermal expansion—arising from the enhanced vibrations of positionally disordered ligand atoms—which effectively counterbalance the intrinsic positive thermal expansion. Moreover, CASO maintains structural integrity up to 1,100 K, features a solar-blind ultraviolet transparency window down to 275 nm and exhibits thermally induced optical fluctuations at least twice as low as those of conventional optical materials. This work provides both a high-performance crystal for extreme thermal environments and a strategy for designing wide-temperature-range ZTE materials. Phonon engineering of zero thermal expansion materials faces a trade-off between open frameworks, which can enable negative thermal expansion through transverse vibrations but can suffer from thermal instability, and closed frameworks, which are more robust yet exhibit weaker negative thermal expansion. Now, fractional atomic occupancy in sodalite-type closed frameworks is shown to exhibit isotropic zero thermal expansion over a wide temperature range.