Vibrational Response of Methylammonium Lead Iodide: From Cation Dynamics to Phonon–Phonon Interactions

Abstract

The dynamic evolution of the vibrational interactions in the prototypical CH NH PbI was studied through a comprehensive experimental and theoretical investigation with a focus on the interactions between the organic cations and the inorganic cage. To date, no clear picture has emerged on the critical and fundamental interactions between the two perovskite components, despite the relevance of phonons to the electronic properties of several classes of perovskites. For the first time, we have monitored the IR and nonresonant Raman response in the broad frequency range 30-3400 cm and in the temperature interval 80-360 K. Strong changes in the energies of different vibrational modes with temperature are observed and examined in the framework of phonon-phonon interactions considering a significant anharmonic contribution to the phonon relaxation process. The vibrational relaxation of the bending modes and their reorientation activation energies identify that such mechanisms are governed by medium-to-strong hydrogen bonds in the orthorhombic phase; however, any ferroelectric ordering in the orthorhombic phase is governed mostly by dipole interactions. These changes imply that charge localization mechanisms play a primary role, and our study enriches the fundamental knowledge of phonon interactions and charge transport in CH NH PbI for the further development of optoelectronic applications.

Infrared Spectroscopic Study of Vibrational Modes across the Orthorhombic–Tetragonal Phase Transition in Methylammonium Lead Halide Single Crystals

“…The spectra are quite similar for the room temperature data as w ell as for the low temperatures. The 80 K (blue) spectre from Ivanovska et al [14] is somewhat sharper in the 3000 -3300 cm -1 frequency range than our spectra at 107 K.…”

“…Comparison of temperature-dependent infrared spectra of MAPbI 3 based on literature data. Red and blue: experimental IR spectra from Ivanovska et al [14] (Figure taken from figure S1 in [14]) in comparison with our infrared spectra (black line: 293 K; green line: 107 K). The spectra are quite similar for the room temperature data as w ell as for the low temperatures.…”

“…b) 107 K measurement of small single crystals using a FTIR -microscope spectrometer (black line); for comparison blue line: experimental IR spectra of MAPbI 3 thin film from Ivanovska et al (scaled data from Fig. 2a from reference [14]). Red arrows: combination modes not assigned in previous works; blue arrow: ν 10 discussed in text.…”

“…14 It should be emphasized that infrared absorption spectroscopy is the method of choice for studying these materials, since it is not subject to the degradation issues 45,46 that were observed in Raman spectroscopic experiments when a photon excitation energy is used that is not well below the optical transition. 14,17,19 6…”

“…S9 a, c, e), ν 10 was assigned to a low frequency shoulder that could be observed for all halides (Lorentzian i10 at 1451 cm -1 for MAPbI 3 , br10 at 1455 cm -1 for MAPbBr 3 , and cl12 at 1455 cm -1 for MAPbCl 3 ) and ν 3 was assigned to the main peak at higher frequencies (Lorentzians i11 at 1469 cm -1 for MAPbI 3 , br12 at 1478 cm -1 for MAPbBr 3 , and cl15 at 1485 cm -1 for MAPbCl 3 ). In recent publications [14][15][16][17]21 the assignment of the fundamental frequency ν 10 did not show a consistent picture, mainly because the shoulder at around 1455 cm -1 was ignored. Visual comparison of our data with published results clearly show the appearance of ν 10 in all reported spectra at the frequencies proposed here ( Fig.…”