X-Ray and Mechano-Induced Valence Transition and Luminescence of Ln³⁺/Ln²⁺ in CsCaCl₃

Lanthanide (Ln)-doped perovskites show immense potential in luminescence. Although Ln²⁺ ions offer superior luminescence efficiency and spectral tunability over Ln³⁺, realizing Ln²⁺ luminescence remains a formidable challenge. Here, a novel strategy based on reduction potentials of 12 Ln³⁺ ions is developed to achieve selective reduction of Ln³⁺ to Ln²⁺ in CsCaCl₃ using x-rays and mechanical force. Specifically, ions with lower reduction potentials (Eu³⁺, Yb³⁺, Sm³⁺) are reduced to the divalent state, whereas those with higher reduction potentials remain trivalent. Notably, the photoluminescence of Eu²⁺ increases by two orders of magnitude after x-ray irradiation. Meanwhile, non-reducible Ln³⁺ ions exhibit ultra-long persistent luminescence from the ultraviolet to near-infrared region, with Tb³⁺ showing a persistence time of 98 s (decay to 1/10 of its initial intensity) outperforming most commercial materials. Moreover, both Ln²⁺ and Ln³⁺ in CsCaCl₃ 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.