Temperature-Dependent Evolution of Raman Spectra of Methylammonium Lead Halide Perovskites, CH₃NH₃PbX₃ (X = I, Br)

We present a Raman study on the phase transitions of organic/inorganic hybrid perovskite materials, CH₃NH₃PbX₃ (X = I, Br), which are used as solar cells with high power conversion efficiency. The temperature dependence of the Raman bands of CH₃NH₃PbX₃ (X = I, Br) was measured in the temperature ranges of 290 to 100 K for CH₃NH₃PbBr₃ and 340 to 110 K for CH₃NH₃PbI₃. Broad ν₁ bands at ~326 cm^-1 for MAPbBr₃ and at ~240 cm^-1 for MAPbI₃ were assigned to the MA⁻PbX₃ cage vibrations. These bands exhibited anomalous temperature dependence, which was attributable to motional narrowing originating from fast changes between the orientational states of CH₃NH₃⁺ in the cage. Phase transitions were characterized by changes in the bandwidths and peak positions of the MA⁻cage vibration and some bands associated with the NH₃⁺ group.