Melting Point Depression and Glass-Forming Abilities in Pb- and Sn-Based Hybrid Perovskites with a Nonprimary Ammonium Cation

Abstract Two-dimensional organic–inorganic perovskites can attain lower congruent melting temperatures (Tm) through compositional design, including, for example, by incorporating nonprimary ammonium cations, enabling solvent-free melt processing and glass formation. Although Pb- and Sn-based systems have been explored, the structural and mechanistic origins of their distinct thermal behaviors remain poorly understood. Here, starting from the model (MC3I)2PbI4 (MC3I = N-methyliodopropylammonium), we demonstrate that replacing Pb with Sn lowers Tm to 86.0 °C (359.2 K) and markedly modulates glass-forming ability. Temperature-dependent single-crystal X-ray diffraction, pair distribution function, and first-principles electron localization functions provide continuous structural insights from 100 K to the molten state, enabling direct comparison across the melting transition and correlation with thermal behavior. We demonstrate that Sn substitution yields larger structural distortions (e.g., associated with increased lone-pair stereoactivity), selectively elongated and more ionic Sn–I bonds, reduced tolerance for atomic thermal vibrations, and more accessible atomic arrangements in the melt, resulting in a higher melting entropy (ΔSm) and lower Tm. Both Pb and Sn compounds exhibit robust melt stability over 50 crystal-melt-glass cycles without discernible decomposition, while showing increased glass-forming propensity. Prolonged melt dwelling effectively promotes vitrification in the Sn compound, despite its relatively lower glass-forming ability relative to the Pb-based system.