Zt. Jiang et al., INVESTIGATION OF THE FERROELECTRIC PHASE-TRANSITION IN (CH3NH3)HGCL3, Journal of physics and chemistry of solids, 56(2), 1995, pp. 277-283
The ferro- to para-electric structural phase transition (T(c) = 60 +/-
2-degrees-C) in methylammonium trichloromercurate(II) has been invest
igated using XPS, IR, and DSC. Core level binding energy shifts of N(1
s), Cl(2p) and Hg(4f), from XPS spectra, indicate that the chemical en
vironments of these atoms are totally different in the two phases. The
shifts are also compared with those calculated based on the restructu
ring which accompanies the phase transition. The observed infrared tem
perature 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, t
he N-H stretching modes show little temperature dependence through T(c
). These results support the notion that H-bonding is the principal ca
use of moment orientation in the ferroelectric phase but is not strong
ly diminished by the phase transition.