<p>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.</p><p></p>