Multicolor and Hydrochromic Luminescence of Alloyed Cs ₂ CdCl ₄ for Advanced Information Encryption

Abstract Advanced optical information encryption usually relies on the integration of multicolor, stimulus‐responsive, and afterglow luminescence. However, achieving these simultaneously within a single‐phase system has posed challenges, complicating the design process. In this study, an advanced 3D information encryption strategy is presented based on undoped and alloyed Cs₂CdCl₄, encrypted through three distinct channels: emission wavelength, water stimulation, and afterglow time. Specifically, Mn 2+ , Sb 3+ , and Ag + doped Cs₂CdCl₄ single crystals are successfully synthesized via a modified hydrothermal method, which exhibit bright orange, green, and blue emissions, respectively. Decryption of specific information can be achieved by employing corresponding narrowband filters. Furthermore, the Cs₂CdCl₄ system demonstrates hydrochromism, attributed to water‐induced sequential phase transitions. Upon water treatment, the 2D Cs₂CdCl₄ will undergo a phase transition to Cs 3 Cd 2 Cl 7 , and finally to cubic CsCdCl 3 . Interestingly, the resultant CsCdCl 3 exhibits pronounced afterglow, originating from the triplet emission of Cd ions. Leveraging these unique properties, a highly secure and adjustable 3D encrypted quick response code is designed. The encrypted information is encoded in coordinates, and decrypted information can be retrieved in both compressed and decompressed modes as required. These findings pave the way for the development of novel smart photonic materials and multimodal optical information encryption.