Halogen-NH₂ ⁺ Interaction, Temperature-Induced Phase Transition, and Ordering in (NH₂CHNH₂)PbX₃ (X = Cl, Br, I) Hybrid Perovskites

Hybrid organic-inorganic FAPbX₃ perovskites (FA = NH₂CHNH₂ ⁺, X = Cl^-, Br^-, I^-) are currently intensively investigated in solar cells. In this study, off-resonance Raman and far-IR absorption spectra of all single-halogen FAPbX₃, as well as of mixed-halogen FAPbBr₂X derivatives, are reported and analyzed. Vibrations of the PbX₆ octahedra and librations of the FA cation lay at frequencies below 250 cm^-1, while external and internal FA bands are identified above 200 cm^-1. An increase in the frequencies of most vibrational bands is observed upon substituting lighter and more electronegative halides for heavier ones due to strengthening of the electrostatic interaction between X^- and the FA NH₂ ⁺ groups. Variable-temperature Raman measurements are also carried out for FAPbBr₃ and FAPbCl₃ in the 77-400 K range. Upon cooling, the three Pb-X Raman vibrations of the cubic structure, split into eight separate bands, signaling the transformation to the tetragonal phase at âˆ240 K and âˆ200 K for FAPbBr₃ and FAPbCl₃, respectively. At even lower temperatures, a successive phase transformation to low symmetry orthorhombic phases is evidenced. These temperature-induced effects are accompanied by intense narrowing, soft-mode behavior, and/or sudden frequency shift of specific Raman bands, attributed to ordering of the FA cation.