Abstract
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.
Highlights
Extensive studies show that cyanides are attractive switchable dielectric and ferroelectric materials,[8−10] dicyanamides are photoluminescent, nonlinear optical (NLO), and barocaloric multiferroic materials,[11−15] while formates have attracted a lot of interest due to their multiferroic properties.[16,17]
We have shown that MHyPbBr3 exhibits second-harmonic generation (SHG) activity, thermochromism, two-photon upconverted PL, and switchable dielectric properties,[23] while MHyPbCl3 exhibits the broadband white PL and quadratic NLO switching behavior.[24]
The internal modes of all halide perovskites exhibit shift to lower wavenumbers when a smaller Cl− ligand is replaced by larger Br− and I−. We have attributed this behavior to weakening of the organic cation confinement with the increasing size of the halide ligand
Summary
Hybrid organic−inorganic compounds (HOIPs) have attracted enormous interest in recent years due to their functional and tunable properties.[1−5] One of the most important subgroup of HOIPs is the family of three-dimensional (3D) perovskites of general formula ABX3, where A stands for the organic cation (typically protonated amine), B is the divalent metal ion (for instance, Pb2+, Sn2+, Mn2+, or Zn2+), and X stands for the organic or inorganic anion (halide, formate, hypophosphite, cyanide, dicyanamide, azide, thiocyanate).[1,6,7] Extensive studies show that cyanides are attractive switchable dielectric and ferroelectric materials,[8−10] dicyanamides are photoluminescent, nonlinear optical (NLO), and barocaloric multiferroic materials,[11−15] while formates have attracted a lot of interest due to their multiferroic properties.[16,17] The most extensively studied 3D perovskites are, halides. Temperature-dependent structural changes of perovskites were studied by various experimental (X-ray diffraction, IR, Raman, EPR, NMR, dielectric spectroscopy, etc.)[25−28] and theoretical methods.[29,30] In particular, to obtain deep insight into the structure−property relationship in hybrid perovskites, it is important to study their phonon properties since such studies provide information on the strength of chemical bonds, crystal symmetry, symmetry, and dynamics of molecular units.[27,31,32] vibrational spectroscopy is a very sensitive tool that can be widely used for studies of light-atom dynamics.
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