Melting of an anchored bilayer: molecular dynamics simulations of the structural transition in (C_nH_2n+1NH₃)₂PbI₄ (n = 12, 14, 16, 18)

Abstract

The thermally driven structural phase transition in the organic-inorganic hybrid perovskite (C_nH_2n+1NH₃)₂PbI₄ has been investigated using molecular dynamics (MD) simulations. This system consists of positively charged alkyl-amine chains anchored to a rigid negatively charged PbI₄ sheet with the chains organized as bilayers with a herringbone arrangement. Atomistic simulations were performed using an isothermal-isobaric ensemble over a wide temperature range from 65 to 665 K for different alkyl chain lengths, n = 12, 14, 16, and 18. The simulations are able to reproduce the essential features of the experimental observations of this system, including the existence of a transition, the linear variation of the transition temperature with alkyl chain length, and the expansion of the bilayer thickness at the transition. By use of the distance fluctuation criteria, it is shown that the transition is associated with a melting of the alkyl chains of the anchored bilayer. An analysis of the conformation of the alkyl chains shows increased disorder in the form of gauche defects above the melting transition. Simulations also show that the melting transition is characterized by the complete disappearance of all-trans alkyl chains in the anchored bilayer, in agreement with experimental observations. A conformationally disordered chain has a larger effective cross-sectional area, and above the transition a uniformly tilted arrangement of the anchored chains can no longer be sustained. At the melt the angular distribution of the orientation of the chains are no longer uniform; the chains are splayed allowing for increased space for individual chains of the anchored bilayer. This is reflected in a sharp rise in the ratio of the mean head-to-head to tail-to-tail distance of the chains of the bilayer at the transition resulting in an expansion of the bilayer thickness. The present MD simulations provide a simple explanation as to how changes in conformation of individual alkyl-chains gives rise to the observed increase in the interlayer lattice spacing of (C_nH_2n+1NH₃)₂PbI₄ at the melting transition.