Blue‐Ultraviolet‐X‐Ray Ultra‐Wideband Activated Photochromism and Multi‐Type Fluorescence in SrGd ₂ O ₄ :Bi ³⁺ for Versatile Applications

Abstract Photochromic materials integrating multi‐band excitation and multi‐type fluorescence exhibit remarkable advantages in information storage, anti‐counterfeiting, X‐ray imaging, and beyond. However, the number of such materials remains limited due to the lack of development strategies. Herein, a lanthanide oxide, SrGd ₂ O ₄ is employed, characterized by a low‐dimensional tubular crystal structure as the matrix, which facilitates the generation of abundant defects and efficient X‐ray absorption. By introducing Bi ³⁺ ions featuring broadband absorption, strong fluorescence, and easy valence change, the coloration‐inducing wavelengths are extended from 254 nm and X‐ray to 365 nm and blue light. The photochromic response is fully reversible, with the colored state efficiently erased by white light irradiation or thermal stimuli. Notably, two distinct photochromic mechanisms are identified: oxygen vacancy‐related color centers and an absorption band arising from valence variation of Bi ³⁺ ions. Additionally, SrGd ₂ O ₄ : Bi ³⁺ displays blue photoluminescence and high‐temperature afterglow under 254 nm/X‐ray excitation. These combined properties enable the material to achieve advanced applications in bright‐and‐dark‐field X‐ray imaging, daylight‐activated optical anti‐counterfeiting, and rewritable information storage systems.