Temperature-Dependent Evolution of Raman Spectra of Methylammonium Lead Halide Perovskites, CH(3)NH(3)PbX(3) (X = I, Br)

We present a Raman study on the phase transitions of organic/inorganic hybrid perovskite materials, CH(3)NH(3)PbX(3) (X = I, Br), which are used as solar cells with high power conversion efficiency. The temperature dependence of the Raman bands of CH(3)NH(3)PbX(3) (X = I, Br) was measured in the temperature ranges of 290 to 100 K for CH(3)NH(3)PbBr(3) and 340 to 110 K for CH(3)NH(3)PbI(3). Broad ν(1) bands at ~326 cm(−1) for MAPbBr(3) and at ~240 cm(−1) for MAPbI(3) were assigned to the MA–PbX(3) cage vibrations. These bands exhibited anomalous temperature dependence, which was attributable to motional narrowing originating from fast changes between the orientational states of CH(3)NH(3)(+) 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 NH(3)(+) group.