Terahertz Spectroscopic Measurements and Solid-State Density Functional Calculations on CH₃NH₃PbBr₃ Perovskites: Short-Range Order of Methylammonium

Abstract

We studied the vibrational absorption spectra of CH3NH3PbBr3 perovskites in the frequency range from 20 to 200 cm–1 at temperatures from 10 to 293 K. The spectrum at 10 K shows several sharp bands from 20 to 120 cm–1, and there is a broad band centered at 155 cm–1. From 40 to 120 cm–1, the spectrum looks like a sum of the sharp bands and a broad background. These broad spectral features are not usually observed at very low temperatures for molecular crystals. We calculated the terahertz (THz) spectrum using solid-state density functional theory and the single unit-cell configuration crystal structure measured by neutron scattering at 11 K. However, this model did not capture the characteristics of the observed spectra, especially for the broad spectral features. Here we propose a structural model in which the methylammonium (MA) cations can take two possible orientations at equal probability, a situation which leads to the coexistence of 16 unit-cell configurations in terms of MA cations’ orientations: Four unit cells conserve centrosymmetry and are nonpolarized, and all the remaining twelve unit cells break centrosymmetry with net dipole moments. By calculating the THz modes associated with the 16 unit-cell configurations under the periodic boundary conditions, we have qualitatively captured the characteristics of the observed THz spectra. Regarding the spatial distributions of the 16 unit-cell configurations, our study suggests that short-range-ordered domains should exist and that the polarized domains can be in the majority.