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

Raman spectra of polycrystalline samples of CH 3 NH 3 PbC1 3 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 PbCI 6 octahedra. A systematic measurement of the CH 3 NH 3 + torsional vibration (at 483 cm -1 ) was performed, and the activation energy E a = 10.1 kJmol -1 was extracted from the half-width versus temperature curve.