Melting of an anchored bilayer: phase transitions in the organic-inorganic hybrid pervoskite (CH₃NH₃)(CH₃(CH₂)_nNH₃)₂Pb₂I₇ (n = 11, 13, 15, 17)

Abstract

The conformation, organization, and phase transitions of alkyl chains in organic-inorganic hybrids based on the double pervoskite-slab lead iodides, (CH₃NH₃)(CH₃(CH₂)_nNH₃)₂Pb₂I₇ (n = 11, 13, 15, 17) have been investigated by X-ray diffraction, calorimetry, and infrared vibrational spectroscopy. In these hybrid solids, double pervoskite (CH₃NH₃)Pb₂I₇ slabs are interleaved with alkyl ammonium chains with the anchored alkyl chains arranged as tilted bilayers and adopting a planar all-trans conformation at room temperature. The (CH₃NH₃)(CH₃(CH₂) _nNH₃)₂Pb₂I₇ compounds exhibit a single reversible phase transition above room temperature with the associated enthalpy change varying linearly with alkyl chain length. This transition corresponds to the melting in two-dimensions of the alkyl chains of the anchored bilayer and is characterized by increased conformational disorder of the methylene units of the chain and loss of tilt angle coherence leading to an increase in the interslab spacing. By monitoring features in the infrared spectra that are characteristic of the global conformation of the alkyl chains, a quantitative relation between conformational disorder and melting of the anchored bilayer is established. It is found that, irrespective of the alkyl chain length, melting occurs when at least 60% of the chains in the anchored bilayer of (CH₃NH₃)(CH₃(CH₂) _nNH₃)₂Pb₂I₇ have one or more gauche defects. This concentration is determined by the underlying lattice to which the alkyl chains are anchored.