Vibrational spectroscopic study of the structural phase transitions in Perovskite layer compounds (CH 3NH 3) 2 CdCl 4

Abstract

The infrared (IR) and Raman spectra of (CH 3NH 3); CdCL 4 [C 1CdCl] are reported in the internal mode region in the 90–300 K temperature range. From an analysis of a few thermosensitive bands in the 800–2000 cm −1 range, it is concluded that C 1CdCl undergoes a transition from a tetragonal low temperature (TLT) phase to a monoclinic low temperature (MLT) phase at 160 ± 2 K. No spectroscopic manitestations are observed for a transition from an orthorhombic room temperature (ORT) phase to the TLT phase which has been reported to occur at 283 K. The observed dissimilar linewidth changes in the ν 11 (NH 3 rocking) and ν 12 (CH 3 rocking) modes around 160 K have been attributed to a dynamical motion of the methylammonium ion. The spectral parameter variations for ν 11 and ν 12 have been explained using a soft-hard mode coupling model and the value of β (order parameter critical exponent) was found to be 0.26 ± 0.05. A change in the hydrogen bonding scheme is clearly shown by the temperature dependence of the IR active mode (ν 6+ ν 9) and by the Raman frequency shifts in the ν 1 and ν 2 modes. From an Arrhenius type plot of the Raman peak intensity ratio variation for two ν 2 components, the activation energy of 845 cal mol −1 has been calculated.