Stepwise Population Accumulation‐Induced Efficient Multi‐Excited‐State Transition Emissions of Eu 3+ in Cs 2 KInCl 6 Double Perovskite Phosphors

ABSTRACT 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.