High‐Entropy Rare‐Earth Halide Double Perovskites Convert Compositional Disorder Into Ion‐Transport‐Stabilized Broadband Near‐Infrared Emission for LEDs

The high‐entropy halide‐perovskite field has expanded rapidly, yet two central chemical questions remain insufficiently understood: how compositional disorder in complex ionic lattices can be converted into predictable, component‐differentiated photophysical behavior with tailorable functionality, and what atomistic origin underlies the enhanced environmental robustness. Here we address these questions using entropy‐engineered rare‐earth halide double‐perovskite single crystals, Cs 2 Na(Sb, RE)Cl 6 (RE 3+ = Sc 3+ , Er 3+ , Yb 3+ , and Tm 3+ ), as a composition‐tunable platform. Near‐equiatomic B(III)‐site alloying yields a single‐phase high‐entropy solid solution (Δ S config ≈ 1.6R), where cations assume complementary, component‐specific photophysical functions. The ns 2 ‐configured Sb 3+ centers provide broadband absorption and sensitization, whereas RE 3+ define orthogonal NIR emissive manifolds. By integrating chemically distinct optical centers within one lattice, compositional disorder is converted from a mere entropy‐stabilization motif into a tailorable emissive architecture, producing multipeak NIR emission across ∼850–1600 nm for self‐referenced ratiometric sensing. Accelerated aging verifies relatively improved phase and emission stability, while combined DFT and MD analyses provide, a mechanistic, simulation‐supported rationalization of high‐entropy stabilization in halide double perovskites: configurational entropy thermodynamically disfavors decomposition, whereas suppressed RE 3+ /Cl − self‐diffusion kinetically retards ion‐migration‐assisted reconstruction and degradation. Together, these results translate role‐differentiated emission into stable broadband NIR LEDs, validating entropy engineering for durable perovskite photonics.