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 ∼240 K and ∼200 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.