Lead‐Free Halide Double Perovskites: A Review of Recent Advances in Synthesis, Optical Properties and Applications

ABSTRACT

Lead‐free halide double perovskites, including Cs ₂ NaInCl ₆ , its lanthanide‐doped derivatives (Cs ₂ NaLnCl ₆ ) and Cs ₂ AgInCl ₆ , have emerged as promising environmentally friendly materials for next‐generation optoelectronic applications. These materials exhibit attractive optical properties arising from self‐trapped excitons and lanthanide ion emissions. Consequently, they can produce light over a broad spectral range of 400–2000 nm and exhibit enhanced photoluminescence quantum yields (PLQYs) in optimized doped systems. Their emission colour can also be tuned from warm to cool white by incorporating dopants such as Na ⁺ , Bi ³⁺ and other lanthanide ions. Despite these advantages, significant challenges remain in achieving large‐scale synthesis and precise control over the incorporation of multiple lanthanide ions, both of which are essential for practical applications in lighting, sensors, photocatalysis and X‐ray detection while ensuring long‐term stability and environmental safety. This review provides a comprehensive overview of the synthesis methods, crystal structures, optical properties, emission tuning mechanisms and emerging applications of Cs ₂ NaInCl ₆ , Cs ₂ NaLnCl ₆ and Cs ₂ AgInCl ₆ . It also summarizes recent advances, discusses the key challenges and outlines future research directions for the development of practical lead‐free double‐perovskite phosphors.