Iodine–Iodine Interactions Suppressing Phase Transitions of 2D Layered Hybrid (I-(CH₂)_n-NH₃)₂PbI₄ (n = 2–6) Perovskites

Abstract

Two-dimensional layered hybrid perovskites like (H-(CH₂)_6-NH₃)₂PbI₄ have hydrogen bonding interactions between organic and inorganic sublattices, influencing structure and properties. When a halogenated organic cation is used forming layered perovskites like (I-(CH₂) _6-NH₃)₂PbI₄, a new noncovalent interaction between the iodine tail of the organic cation and iodine of the Pb–I inorganic sublattice is introduced. How does the new iodine–iodine interaction, in conjunction with hydrogen bonds, modulate the crystal structure and optical properties of layered hybrid perovskites? Here we address this question providing new insights and functionality. We prepared single crystals of (I-(CH₂)_n-NH₃) ₂PbI₄ (n = 2–6) and measured single-crystal X-ray diffraction and photoluminescence at different temperatures. All the samples (n = 2–6) do not show a temperature-dependent phase transition in the range of 10 to 296 K (approximately room temperature), in contrast to samples containing nonhalogenated organic cations. Then we introduce a structural parameter (r) indicating symmetry of hydrogen bonding (−NH3 group) with respect to the inorganic sublattice. Lower values of r indicate more symmetry in hydrogen bonding, leading to a higher Pb–I–Pb bond angle and lower band gap for n = 5–6. Higher r increases asymmetry in hydrogen bonding, decreasing the band gap for n = 2–4.