Nearly‐Unity Quantum Yield and 12‐Hour Afterglow from a Transparent Perovskite of Cs2NaScCl6:Tb

Abstract Unlike afterglow phosphors, transparent crystals free of scattering issues hold great potential in applications such as volumetric displays and three‐dimensional encryption. Here, a double‐perovskite host of ca. 85 % transparency, a Cs 2 NaScCl 6 single crystal, was grown in aqueous solution. Through heavy doping of Tb ions, the crystal exhibited a high photoluminescence quantum yield (PL QY) up to 98.2 %. Both steady‐state and transient PL spectroscopic investigation revealed a cross‐relaxation efficiency ( η CR ≈99 %) between Tb ions, greatly mitigated the electron saturation on the 5 D 3 level and encouraged the radiative route against quenching. The doped crystal displayed an intense afterglow of up to 12 h after ceasing the X‐ray excitation, allowing it to be used in a radiation‐storage battery which has a linear scaling capacity with loading weight. Our work opens possibilities in photo‐stimulated volumetric display and three‐dimensional information storage.