Raman study of low temperature phase transitions in the cubic perovskite CH₃NH₃PbCI₃

Abstract

Abstract Raman spectra of polycrystalline samples of CH3NH3PbC13 were studied at between 80 and 300 K. An assignment of most of the observed bands is proposed. The first-order phase transitions previously detected at 172 and 179K were characterized: The spectral evolution and the mechanism proposed are in agreement with the theoretical analysis of normal vibrations and with previous X-ray diffraction studies and dielectric measurements. The I ⌆ II transition involves mainly conformational and orientational motions of the organic cation, whereas the antiferroelectric transition at 172K is connected with a complex mechanism involving the freezing of the cation motions coupled with the distortion of the PbCI6 octahedra. A systematic measurement of the CH3NH3 + torsional vibration (at 483 cm–1) was performed, and the activation energy Ea= 10.1 kJmol–1 was extracted from the half-width versus temperature curve.