Effect of halogen substitution in spacer cations on two-dimensional Ruddlesden–Popper perovskites

This research investigated how halogen substitution in spacer cations influenced the crystal and electronic structures of two-dimensional Ruddlesden-Popper perovskites (X-(CH₂)₂-NH₃)₂PbI₄ (where X can be I, Br, or Cl) through first-principle calculations. In contrast to (I-(CH₂)₂-NH₃)₂PbI₄, the [PbI₆]^4- octahedra in (Br-(CH₂)₂-NH₃)₂PbI₄ and (Cl-(CH₂)₂-NH₃)₂PbI₄ exhibited a greater degree of deviation from ideal octahedral geometry. Moreover, the Pb-I-Pb bond angles in (Br-(CH₂)₂-NH₃)₂PbI₄ and (Cl-(CH₂)₂-NH₃)₂PbI₄ were approximately 180°, indicating that the distortions of the adjacent [PbI₆]^4- octahedra were relatively minor. In comparison, (I-(CH₂)₂-NH₃)₂PbI₄ demonstrated more significant adjacent [PbI₆]^4- octahedral distortions and therefore a larger band gap. The significant distortion of adjacent [PbI₆]^4- octahedra in (I-(CH₂)₂-NH₃)₂PbI₄ was found to be predominantly induced by hydrogen-bonding interactions between the organic and inorganic components, along with the I-I interactions. In (Br-(CH₂)₂-NH₃)₂PbI₄ and (Cl-(CH₂)₂-NH₃)₂PbI₄, the hydrogen-bonding interactions between the spacer cations facilitated the insertion of two carbon atoms into the pocket of the inorganic layer, leading to significant distortion of the individual [PbI₆]^4- octahedra. Additionally, these hydrogen-bonding interactions significantly contributed to their increased thermal stability. Moreover, the symmetrical positioning of the spacer cations in relation to the inorganic layer led to symmetrical hydrogen-bonding interactions between the organic and inorganic components, which helped prevent the deformation of adjacent [PbI₆]^4- octahedra. This research provided valuable theoretical insights for the selection of organic cations of two-dimensional organic-inorganic hybrid materials.