Effect of Halogen Substitution in Spacer Cations on Two-Dimensional Ruddlesden-Popper Perovskites

Abstract

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-principles calculations. In contrast to (I-(CH2)2-NH3)2PbI4, (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 larger band gap. The significant distortion of adjacent [PbI6]4- octahedra in (I-(CH2)2-NH3)2PbI4 was found to 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.