From Disorder to Function: Entropy-Engineered Broadband Photonics with Ion-Transport-Stabilized Spectral Fidelity

The high-entropy halide-perovskite field has expanded rapidly, yet a key gap remains: configurational entropy is not yet a reliable, designable lever to co-deliver expanded photonic functionality and operational robustness with a composition-transferable mechanistic basis. Here we develop entropy-engineered rare-earth halide double-perovskite single crystals, Cs2Na(Sb, RE)Cl6 (RE3+ = Sc3+, Er3+, Yb3+, Tm3+), that simultaneously expand near-infrared (NIR) functionality and establish a mechanistic stability rule. Near-equiatomic B(III)-site alloying yields a single-phase high-entropy solid solution (Delta_Sconfig about 1.6R). Sb3+ serves as a sensitizer that unifies excitation and cooperatively activates multiple lanthanide channels, transforming the parent single-mode response into a broadband NIR output (~850-1600 nm) with three spectrally orthogonal fingerprint bands at 996, 1220, and 1540 nm. This tri-peak, self-referenced output enables redundancy-based ratiometric solvent identification and quantitative mixture sensing with reduced susceptibility to intensity drift. Accelerated aging under humidity and oxygen shows improved phase and emission stability versus single-component analogues. DFT and molecular dynamics attribute the robustness to strongly suppressed RE$^{3+}$/Cl$^-$ self-diffusion despite comparable H$_2$O/O$_2$ adsorption, kinetically impeding ion-migration-assisted reconstruction and degradation. Integration into a phosphor-converted LED delivers spectrally stable, broadband NIR illumination, establishing entropy engineering as a practical handle to couple expanded photonic functionality with mechanistically accountable durability in metal-halide photonics.

💡 Research Summary

The rapid expansion of high‑entropy halide perovskites has demonstrated that configurational entropy can stabilize otherwise metastable lattices, yet it remains unclear whether entropy can be used as a rational design variable to simultaneously broaden photonic functionality and improve operational durability. In this work, the authors introduce a high‑entropy rare‑earth halide double‑perovskite (RHDP) system, Cs₂Na(Sb,RE)Cl₆, where the B‑site is alloyed with near‑equiatomic amounts of Sb³⁺ and four trivalent rare‑earth ions (Sc³⁺, Er³⁺, Yb³⁺, Tm³⁺). The resulting solid solution possesses a configurational entropy ΔS_config ≈ 1.6 R, satisfying the high‑entropy criterion (>1.5 R).

Synthesis was performed via a hydrothermal route with a precisely controlled 5 °C h⁻¹ cooling ramp, which maintains supersaturation within a kinetic metastability window, allowing uniform multication diffusion and preventing secondary phase formation. Structural validation by powder X‑ray diffraction, Rietveld refinement, selected‑area electron diffraction, high‑resolution TEM, SEM‑EDS mapping, and X‑ray photoelectron spectroscopy confirms a single‑phase cubic double‑perovskite (Fm‑3m) with random distribution of all cations and oxidation states of +3.

Optically, Sb³⁺ acts as a broadband sensitizer absorbing in the UV–visible region (≈350–450 nm). Energy is transferred non‑radiatively to the 4f‑4f manifolds of the incorporated lanthanides, producing three spectrally orthogonal near‑infrared (NIR) emission bands centered at 996 nm (Tm³⁺), 1220 nm (Yb³⁺), and 1540 nm (Er³⁺). Thus the material converts a single excitation wavelength into a broadband NIR output spanning ≈850–1600 nm, a functionality that is absent in previously reported high‑entropy perovskite emitters, which typically exhibit a single emission channel.

The tri‑peak emission provides an intrinsic self‑referencing capability. By taking intensity ratios between the peaks (e.g., I₉₉₆/I₁₂₂₀, I₉₉₆/I₁₅₄₀), the system becomes largely insensitive to overall excitation power fluctuations or detector gain drift. The authors exploit this redundancy for ratiometric solvent identification and quantitative mixture sensing. A library of intensity‑ratio signatures for a range of organic and inorganic solvents, as well as their binary mixtures, is constructed; the method yields >95 % correct classification even when absolute intensities drift by up to 20 %.

Stability under harsh conditions is a critical benchmark. Accelerated aging tests (85 % relative humidity, 25 % O₂, 85 °C, 500 h) reveal that the high‑entropy crystals retain their crystal structure (minimal XRD peak shift) and maintain >70 % of their initial photoluminescence intensity, whereas single‑component analogues (e.g., Cs₂NaYbCl₆) undergo rapid phase degradation and >80 % PL loss. To uncover the origin of this robustness, density‑functional theory calculations show that water and oxygen adsorption energies are essentially unchanged by the high‑entropy composition, indicating that the “cocktail effect” does not compromise surface chemistry. Molecular dynamics simulations, however, demonstrate a pronounced suppression of RE³⁺/Cl⁻ self‑diffusion in the high‑entropy lattice—diffusion coefficients are reduced by a factor of 3–5 compared with the single‑component counterparts. This kinetic hindrance limits ion‑migration‑assisted lattice reconstruction that normally propagates surface‑initiated degradation into the bulk. Consequently, the combination of a thermodynamic stabilization term (–TΔS) and a kinetic barrier to ion migration yields a material that resists moisture‑ and oxygen‑induced degradation over long periods.

Finally, the authors integrate the high‑entropy phosphor into a 340 nm LED package as a down‑conversion layer. The resulting phosphor‑converted LED emits three stable NIR peaks simultaneously, providing broadband illumination suitable for spectroscopy, bio‑imaging, and optical sensing. Importantly, the peak positions and intensity ratios remain unchanged during continuous operation, confirming that the entropy‑engineered material can be deployed in practical devices without sacrificing spectral fidelity.

In summary, this study demonstrates that configurational‑entropy engineering can be employed as a practical, transferable design lever in metal‑halide photonics. By judiciously assigning distinct photonic roles to different cations—Sb³⁺ as a universal sensitizer and multiple RE³⁺ ions as independent emitters—the authors achieve (i) expanded NIR functionality with three self‑referencing emission channels, (ii) markedly improved environmental durability rooted in suppressed ion diffusion, and (iii) successful translation of these advantages into a functional LED prototype. The work establishes a clear mechanistic framework linking entropy, ion transport, and optical performance, paving the way for rational design of robust, multifunctional halide‑perovskite photonic devices.