Lead‐Free Rare‐Earth Based Halide Double Perovskites: From Fundamentals, Progress to Perspectives

Abstract Lead‐free halide double perovskites are rapidly gaining prominence as a nontoxic, highly stable, and versatile alternative to traditional lead‐based perovskites, notable for their superior optoelectronic properties. Among these, rare‐earth‐based halide double perovskites (RHDPs) notably stand out. Comprising fifteen lanthanide elements with distinctive electron configurations of [Xe] 4f n−1 5d 0−1 6s 2 ( n = 1–15), alongside the group‐IIIB elements Yttrium (Y) and Scandium (Sc), rare‐earth elements (RE) exhibit great potential. These 17 RE ions (Sc 3+ , Y 3+ , and La 3+ –Lu 3+ ) offer a vast landscape for crafting numerous RHDPs, poised to exhibit properties that surpass and diversify beyond current double perovskite standards. Key to their functionality are unique luminescence properties driven by self‐trapped exciton (STE) recombination, and 4f→4f and 5d→4f transitions, enabling applications in visible and infrared light emission through downshifting and up‐conversion. This review thoroughly articulates the foundational properties of RHDPs, including their structural‐property relationships, synthesis methods, optical characteristics, and durability. It highlights recent advancements in their applications across a range of fields such as visible and near‐infrared light emitting diodes (Vis&NIR LEDs), sensors, and anti‐counterfeiting technologies. Additionally, the review discusses the prevailing challenges associated with RHDP materials and their applications, offering insights and future directions.