Kinetics‐Tunable Hydrochromic Luminescence Switching in Rb₃TbF₆:Eu³⁺ Perovskite

Hydrochromic luminescent materials that are responsive to water or moisture have attracted considerable attention owing to their extensive use in sensing and information security. However, fine‐tuning hydrochromic response to water content remains unfulfilled research, posing constraints in the widespread application of hydrochromic luminescence. This study reports a 0D Rb 3 TbF 6 perovskite with highly controllable hydrochromic characteristics. The Rb 3 TbF 6 crystals exhibit a distinct hydrochromic luminescence switching after B‐site doping with Eu 3+ , stemming from Rb 3 TbF 6 to Rb 2 TbF 5 transformation that triggers intra‐crystal Tb 3+ ‐to‐Eu 3+ energy transfer. Through A‐site alkaline metal alloying, the sensitivity of the perovskite to water stimuli is precisely tuned, rendering distinct responses over a wide range of humidity conditions (RH: 1–90% RH) and water levels (v/v: 0.5–55%). As an added benefit, the hydrochromic crystals also show excellent repeatability and photothermal stability. These advances on hydrochromic materials provide exciting opportunities in water detection, humidity sensing, and multi‐level information encryption.