Phase Transitions in the Anchored Organic Bilayers of Long-Chain Alkylammonium Lead Iodides (CnH₂n₊₁NH₃)₂PbI₄; n = 12, 16, 18

The single perovskite slab alkylammonium lead iodides (C n H 2 n +1 NH 3 ) 2 PbI 4, n = 12, 16, 18, display two phase transitions, just above room temperature, associated with changes in the alkylammonium chains. We have followed these two phase transitions using scanning calorimetry, X-ray powder diffraction, and IR and Raman spectroscopies. We find the first phase transition to be associated with symmetry changes arising from a dynamic rotational disordering of the ammonium headgroup of the chain whereas the second transition, the melting of the chains in two dimensions, is characterized by an increased conformational disorder of the methylene units of the alkyl chains. We examine these phase transitions in light of the interesting optical properties of these materials, as well as the relevance of these systems as models for phase transitions in lipid bilayers.