Phonon Properties and Lattice Dynamics of Two- and Tri-Layered Lead Iodide Perovskites Comprising Butylammonium and Methylammonium Cations-Temperature-Dependent Raman Studies.

Hybrid lead iodide perovskites are promising photovoltaic and light-emitting materials. Extant literature data on the key optoelectronic and luminescent properties of hybrid perovskites indicate that these properties are affected by electron-phonon coupling, the dynamics of the organic cations, and the degree of lattice distortion. We report temperature-dependent Raman studies of BA 2 MAPb 2 I 7 and BA 2 MA 2 Pb 3 I 10 (BA = butylammonium; MA = methylammonium), which undergo two structural phase transitions. Raman data obtained in broad temperature (360-80 K) and wavenumber (1800-10 cm -1 ) ranges show that ordering of BA + cations triggers the higher temperature phase transition, whereas freezing of MA + dynamics occurs below 200 K, leading to the onset of the low-temperature phase transition. This ordering is associated with significant deformation of the inorganic sublattice, as evidenced by changes observed in the lattice mode region. Our results show, therefore, that Raman spectroscopy is a very valuable tool for monitoring the separate dynamics of different organic cations in perovskites, comprising "perovskitizer" and interlayer cations.