No abstract

Phase transitions in triammonium hydrogen disulphate (TAHS), occurring below room temperature, are investigated by measuring polarized Raman scattering. A careful examination of changes in frequency, intensity, and width of thermosensitive bands near the transition temperatures is helpful in deciding about the type of phase transition. The value of the order parameter exponent β (0.45 ± 0.03) as estimated by us for the transition at 265 K is consistent with the value for a second‐order transition in the mean field theory approximation. The transitions at 265 and 140 K are found to be of second order, whereas the one at 46 K shows features of a first‐order transition. A coupling of ammonium and sulphate ion translational modes with a soft mode becomes evident on cooling the crystal to 265 K. The increase in ordering of the ammonium ion sublattice seems to be mainly responsible for the transitions at 265 and 140 K. It is determined that hydrogen bonding is strongest in the ferroelectric phase (T < 46 K). The rotation of the SO(2) bond around 46 K appears to be involved in bringing about the transition to the ferroelectric state.

No abstract

Polarized Raman spectra of methylammoniummercury trichloride crystals were measured at room temperature. Factor group analysis involving the correlation method was applied. The observed spectra were interpreted in terms of internal and external oscillations of the constituent molecular ions, CH,NH,+ and HgC1,-.