Water-triggered optical response in Tb3+-doped CaSnF6 crystals for high-security anti-counterfeiting application

Hydrochromic luminescent materials that exhibit remarkable color changes in response to water have attracted significant attention due to their emerging applications. Currently, most research on hydrochromic materials focuses on reversible systems, while irreversible ones based on lead halide perovskites suffer from issues such as environmental toxicity and poor stability. Here, we report novel and environmentally friendly Tb3+-doped CaSnF6 double perovskite crystals that undergo an irreversible phase transition in response to water and show a noticeable increase of green luminescence intensity (similar to 6-fold). Mechanistic investigation reveals that the exposure to water induces a phase transition via a Sn-F bond stripping process, resulting in the transformation of the CaSnF6:Tb3+ microcrystals into the CaF2:Tb3+ nanocrystals (NCs). A complete phase transformation occurs when the mass ratio of CaSnF6 : Tb3+ to water is 1 : 10 with a reaction time of 5 min. Notably, the irreversible phase transformation of the CaSnF6:Tb3+ crystals enables the enhanced safety of the initially encoded data because the decoded information cannot go back to its original state and be observed by others again. This work not only advances the understanding of phase conversion of the CaSnF6 crystals but also provides exciting opportunities for high-security anti-counterfeiting applications.