X‐Ray and Mechano‐Induced Valence Transition and Luminescence of Ln 3+ /Ln 2+ in CsCaCl 3

ABSTRACT Lanthanide (Ln)‐doped perovskites show immense potential in luminescence. Although Ln 2+ ions offer superior luminescence efficiency and spectral tunability over Ln 3+ , realizing Ln 2+ luminescence remains a formidable challenge. Here, a novel strategy based on reduction potentials of 12 Ln 3+ ions is developed to achieve selective reduction of Ln 3+ to Ln 2+ in CsCaCl 3 using x‐rays and mechanical force. Specifically, ions with lower reduction potentials (Eu 3+ , Yb 3+ , Sm 3+ ) are reduced to the divalent state, whereas those with higher reduction potentials remain trivalent. Notably, the photoluminescence of Eu 2+ increases by two orders of magnitude after x‐ray irradiation. Meanwhile, non‐reducible Ln 3+ ions exhibit ultra‐long persistent luminescence from the ultraviolet to near‐infrared region, with Tb 3+ showing a persistence time of 98 s (decay to 1/10 of its initial intensity) outperforming most commercial materials. Moreover, both Ln 2+ and Ln 3+ in CsCaCl 3 exhibit bright mechanoluminescence. Mechanistic investigations identify Cs vacancies as hole traps and Cl vacancies as electron traps, governing carrier storage and release. Leveraging these properties, proof‐of‐concept applications are presented in radiation warning, collision detection, and x‐ray imaging. This work establishes a multi‐stimuli‐responsive platform for valence‐selective luminescence, opening new avenues for smart optoelectronic devices.