Mn²⁺-doped Cs₂NaInCl₆ double perovskites and their photoluminescence properties

Lead-free double perovskites are emerging as an excellent candidate material for optoelectronic devices, owing to their unique properties such as widely adjustable band gap and markedly lower toxicity than the lead-halide counterparts. In this work, a series of Mn²⁺-doped Cs₂NaInCl₆ (Mn²⁺:Cs₂NaInCl₆) DP crystals with tailored Mn²⁺-doping concentrations are synthesized. The effects of the reaction temperatures and Mn/(In + Na) ratios in the raw materials on the Mn²⁺ doping concentrations and their photoluminescence (PL) properties are investigated systematically. The resultant DP crystals exhibit well-resolved Mn^{2+ 4}T₁ → ⁶A₁ d − d emission with an optimal PL quantum yield of 16% and an extremely long excited state lifetime up to ~ 17 ms, which is the longest one of Mn²⁺ emission among those of Mn²⁺-doped II–VI semiconductors or perovskite materials ever reported. Furthermore, the temperature-dependent PL and PL decay are studied in the temperatures range from 120 to 480 K. The decay lifetime decreased monotonously with the rise in temperatures, while the PL intensity gradually reaches a maximum and then keeps nearly constant, followed by a sharp decrease. Such abnormal phenomenon is mainly attributed to the combined effect of intrinsic property of octahedrally coordinated Mn²⁺ in perovskite lattice and enhanced electron–phonon coupling at elevated temperature.