INVESTIGATION OF THE FERROELECTRIC PHASE-TRANSITION IN (CH3NH3)HGCL3

The ferro- to para-electric structural phase transition (T(c) = 60 +/- 2-degrees-C) in methylammonium trichloromercurate(II) has been investigated using XPS, IR, and DSC. Core level binding energy shifts of N(1s), Cl(2p) and Hg(4f), from XPS spectra, indicate that the chemical environments of these atoms are totally different in the two phases. The shifts are also compared with those calculated based on the restructuring which accompanies the phase transition. The observed infrared temperature dependence of the splitting of the nu9 (NH3 asym. deformation) mode and the disappearance of the nu6 (C-N torsional) mode around 60-degrees-C in the IR spectra are attributed to a re-orientation of the CH3-NH3 cation through the order-disorder transition. Surprisingly, the N-H stretching modes show little temperature dependence through T(c). These results support the notion that H-bonding is the principal cause of moment orientation in the ferroelectric phase but is not strongly diminished by the phase transition.