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

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Abstract
We investigate the phase behavior of 2D (CxH2x+1NH3)2[(MA,FA)PbI3]n-1PbI4 layered perovskites near room temperature (-20ºC to +100ºC) as a function of the octahedral layer thickness (n = 1,2,3,4), alkylammonium chain length (butyl, pentyl, hexyl), and identity of the small organic cation (methylammonium, formamidinium). Using differential scanning calorimetry and X-ray diffraction, we observe a prominent and reversible first-order phase transition in each of these compounds corresponding to a partial melting transition of the alkylammonium chains separating the lead halide perovskite layers. The melting temperature, Tm, monotonically increases from 10ºC to 77.9ºC to 95.9ºC as the carbon chain length increases from C4 to C5 to C6, but is insensitive to octahedral layer thickness, n. The latent heat of melting, ΔHm, was found to be in the range 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 2D molecular systems.
Funding Information
  • Basic Energy Sciences (DE-SC0019345)

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