Chemistry in the Molten State: Opportunities for Designing and Tuning the Emission Properties of Halide Perovskites

A series of monolayered lead halide hybrid perovskites (HO₂C(CH₂)NH₃)₂PbX₄, named (Cn)₂PbX₄ (n = 4-6, X = Cl, Br), exhibiting a low congruent melting temperature (T_m) (T_m = 130 °C for (C4)₂PbBr₄), high stability in the molten state, and whitish type emission, are reported. From the synthesis in the molten state, rare solid solutions of mixed organic cations (Cn_1-Cn')₂PbX₄ (n, n' = 4-6; X = Cl, Br; 0 ≤ x ≤1) as well as solid solutions of mixed halides (Cn)₂Pb(X_1-X')₄ (n = 4-6; X, X' = Cl, Br; 0 ≤ y ≤1) have been prepared and characterized (thermal behavior, powder X-ray diffraction (PXRD), photoluminescence properties). The impact of substitutions is significant on the thermal properties, lowering the T_m down to 100 °C for (C4)₂Pb(Br_0.25Cl_0.75)₄. The emission properties are slightly tuned in the case of mixed organic cation systems, whereas modifications are more dramatic in the case of mixed halide systems, leading to emission properties through the entire visible region. These results illustrate the great opportunities offered by the congruent melting properties of halide perovskites allowing syntheses in the molten state.