Retainable Bandgap Narrowing and Enhanced Photoluminescence in Mn‐Doped and Undoped Cs₂NaBiCl₆ Double Perovskites by Pressure Engineering

Abstract Pressure engineering can access novel optoelectronic properties and crystal structures in halide perovskite materials with soft lattice. However, such pressure‐induced phenomena are commonly realized under large pressure or difficult to retain at atmospheric pressure, thus severely hindering their prospects in practical applications. Herein, the pressure‐treated Mn‐doped/undoped Cs₂NaBiCl₆ exhibits a largely retained bandgap narrowing of 12.2% relative to its initial state via compression–decompression cycles, along with durable stability at ambient conditions. This abnormal behavior is attributed to the disordered arrangement of inorganic [NaCl₆]⁵⁻/[BiCl₆]³⁻ octahedra, which occurs in the metastable state of double perovskites from structural reconstruction during decompression processes, and is accompanied by a characteristic of long‐range order and short‐range disorder. A remarkable pressure‐induced emission enhancement from Mn²⁺ ions is discovered upon modest compression, especially at <0.5 GPa, which results from the increased energy transfer efficiency from Bi³⁺ to Mn²⁺ ions owing to more overlapped electronic wave functions from each other. This work can be extended to the design and synthesis of new semiconductor materials with enhanced optoelectronic properties.