Comparative Studies of Phonon Properties of Three-Dimensional Hybrid Organic–Inorganic Perovskites Comprising Methylhydrazinium, Methylammonium, and Formamidinium Cations

Three-dimensional hybrid organic–inorganic perovskites with halide, formate, or hypophosphite ligands are promising photovoltaic, light-emitting, and multiferroic materials. Since the properties of these compounds are strongly affected by changes in lattice dynamics, it is of great importance to understand their phonon properties. We report Raman and IR spectra for a number of perovskites to understand the effect of various metal-linker frameworks on vibrations of methylhydrazinium, formamidinium, and methylammonium cations as well as effects of these cations on lattice phonons in lead bromide analogues. Our results show that the lattice dynamics and energy of lattice phonons of lead halides depend strongly on the type of organic cation and temperature. In particular, at room temperature, the dynamics of methylhydrazinium cations is much slower compared to the dynamics of methylammonium and formamidinium cations, implying weaker electron scattering in the former case and thus significantly different optoelectronic properties compared to the formamidinium and methylammonium analogues. We also show that the size of the halide ion affects the energy of internal modes, but this effect is much more pronounced when halide ligands are replaced by formate or hypophosphite anions, especially in the case of formamidinium analogues. We attribute this behavior to strong variation of hydrogen bond strength and changes in the internal structure of organic cations.