Melting Transitions of the Organic Subphase in Layered Two-Dimensional Halide Perovskites

We investigate the phase behavior of two-dimensional (C _xH_{2 x+1}NH₃)₂[(MA,FA)PbI₃] _n-1PbI₄ layered perovskites near room temperature (-20 °C to +100 °C) as a function of the octahedral layer thickness ( n = 1, 2, 3, 4), alkylammonium chain length (butyl, pentyl, and hexyl), and identity of the small organic cation (methylammonium and formamidinium). Using differential scanning calorimetry and X-ray diffraction, we observe a reversible first-order phase transition corresponding to a partial melting transition of the alkylammonium chains separating the perovskite layers. The melting temperature, T_m, increases from 10 to 77.9 to 95.9 °C as the carbon chain length increases from C₄ to C₅ to C₆, but it is insensitive to octahedral layer thickness, n. The latent heat of melting, Δ H_m, was in the range of 3-5 kJ/mol-spacer, indicating only partial disordering of the carbon chain. We discuss these findings and their implications in the context of melting in other two-dimensional molecular systems.