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

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.