Highly Stable and Efficient Multimodal Luminescence in Rare‐Earth‐Based Metal Halides Enabled by High‐Entropy Design

ABSTRACT Rare‐earth‐based metal halides have emerged as promising alternatives to lead‐based perovskites owing to their diverse lanthanide compositions, high structural tunability, and versatile optoelectronic properties. However, their practical development has been hindered by complex synthetic procedures and poor humidity stability. Here, we introduce an organic–inorganic A‐site dual‐cation hybrid strategy to address these challenges and report a family of stable lanthanide‐based metal halides with the general formula (DFPD) 2 CsLnCl 6 (DFPD = 4,4‐difluoropiperidine; Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb). Incorporation of Cs + ions increases lattice rigidity, enhancing moisture and thermal robustness and regulating octahedral distortion. Such structural modulation enables efficient ns 2 ion‐mediated sensitization, yielding near‐unity photoluminescence efficiencies without perturbing the characteristic f ‐ f emissions of the lanthanide centers. Building on this framework, configurational entropy engineering at the B sites affords three high‐entropy rare‐earth‐based metal halides that preserve the optical signatures of individual lanthanide ions and enable customizable multimodal luminescence spanning the ultraviolet, visible, and near‐infrared regions. This work establishes a new class of rare‐earth‐based hybrid metal halides and provides a general design strategy for the controllable synthesis of multimodal, high‐entropy luminescent materials for ultraviolet sterilization, solid‐state lighting, and near‐infrared imaging applications.