Stepwise Population Accumulation-Induced Efficient Multi-Excited-State Transition Emissions of Eu3+ in Cs2KInCl6 Double Perovskite Phosphors

Novel Eu 3+ -doped Cs 2 KInCl 6 double perovskite exhibits ultrabroadband emission, enabling efficient multicolor luminescence covering the entire visible spectrum with a single luminescent center. This behavior originates from the intrinsically low phonon energy of the host, which effectively suppresses multi-phonon relaxation (MPR) and consequently reshapes the radiative pathways of Eu 3+ . As a result, its stepwise population accumulation occurs along the excited-state energy ladder, promoting multiple radiative transitions. Based on this kinetic behavior, the rare coexistence of blue ( 5 D 3 → 7 F 4 ), green ( 5 D 2 → 7 F 3 ), yellow ( 5 D 1 → 7 F 2 ), and red ( 5 D 0 → 7 F 2 ) emissions are achieved with large emission cross-sections of 2.00 × 10 −22 , 4.75 × 10 −22 , 5.48 × 10 −22 , and 8.18 × 10 −22 cm 2 , respectively. These emissions are further validated by high fluorescence branching ratios exceeding 40%, confirming the efficiency of the corresponding radiative channels and their collective contribution to multipeak full-spectrum emission. In addition, the phosphors demonstrate pronounced thermal responsiveness in the temperature range of 303 – 433 K, together with dynamically tunable emission, supporting their applications in high-sensitivity optical thermometry, dynamic anti-counterfeiting, and high-quality lighting. Overall, this work suggests that ultralow-phonon-energy perovskite provides an effective platform for achieving multiple excited-state radiative transitions, thus offering a research foundation for developing functional optical materials and next-generation photonic devices.