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7169196 
Journal Article 
Halogen-NH2+ Interaction, Temperature-Induced Phase Transition, and Ordering in (NH2CHNH2)PbX3 (X = Cl, Br, I) Hybrid Perovskites 
Kontos, AG; Manolis, GK; Kaltzoglou, A; Palles, D; Kamitsos, E; Kanatzidis, MG; Falaras, P; , 
2020 
Yes 
Journal of Physical Chemistry C
ISSN: 1932-7447
EISSN: 1932-7455 
AMER CHEMICAL SOC 
WASHINGTON 
8479-8487 
WOS:000529225800003 
Hybrid organic-inorganic FAPbX(3) 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 FAPbX(3), as well as of mixed-halogen FAPbBr(2)X 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 FAPbBr(3) and FAPbCl(3) 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 similar to 240 K and similar to 200 K for FAPbBr(3) and FAPbCl(3), 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.