Infrared and terahertz studies of phase transitions in the CH3NH3PbBr3 perovskite

We investigated the influence of two structural phase transitions in methylammonium lead tri-bromide CH3NH3PbBr3 (MAPbBr3) single crystals on the activity of polar phonons using time-domain THz transmission and far infrared (IR) reflectance spectra. The fitting using the Lorentz model of damped harmonic oscillators also enables us to determine all the polar phonon parameters, IR response functions, and their temperature dependence. The fit of the far IR spectra provides a nearly perfect quantitative description of the measured reflectance at room temperature using three phonons. On cooling, three IR active phonons of the cubic and tetragonal structures split into 15 separate bands, signaling the transformation to the orthorhombic phase below 150 K. This result is compared with the factor group analysis of the polar phonons in all three crystallographic phases. The absence of a soft optical phonon supports the previously published order-disorder character of the structural phase transitions.