Investigation of the ferroelectric phase transition in (CH₃NH₃)HgCl₃

The ferro- to para-electric structural phase transition (T_c = 60 ± 2 °C) in methylammonium trichloromercurate(II) has been investigated using XPS, IR, and DSC. Core level binding energy shifts of N(ls), 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 v₉ (NH₃ asym. deformation) mode and the disappearance of the v₆ (CN torsional) mode around 60 °C in the IR spectra are attributed to a re-orientation of the CH₃NH₃ cation through the order-disorder transition. Surprisingly, the NH 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.