Studies on phase transitions in crystals

Abstract

The present Raman spectroscopic observations of the mixed crystals of ammonium and potassium sulphate in the paraelectric phase show that as the concentrations of K? ions increase in the crystal, the frequency of totally symmetric vibrations of (SO?)²? increase and at 50% of potassium concentration, there are two frequencies observed at 976 cm?¹ and 990 cm?¹ respectively. This indicates that at that concentration there exist two types of hydrogen bonds, one stronger and the other weaker. The line at 976 cm?¹ is characteristic of the hydrogen-bonded sulphate frequency as observed in pure (NH?)?SO?.

In the ferroelectric phase, as the concentration of K? ions is increased, the slope of the intensity of totally symmetric frequency of SO?²? with temperature varies from a discontinuous change to a continuous change, thereby showing that the first-order character of the phase transition changes to second-order. Also, the degenerate mode of SO?²? vibration gives rise to four bands in the low-temperature phase in (NH?)?SO?, which implies that the sulphate tetrahedron is more distorted in that phase.

Therefore, combining the above results, it is concluded that the ferroelectric phase transition in pure ammonium sulphate is due to the cooperative coupling of NH?? and the distorted (SO?)²? ions in the low temperature through the hydrogen bonds.

The anomalous temperature dependence of (NH?)?SO? may be due to the dipole moment contributions arising from NH?(I), NH?(II), and SO?²? groups, and this could be concluded by a detailed investigation of the domains at 80°K in the mixed crystal system of (NH?)?SO?–K?SO? through electron microscope studies. From the sublattice model point of view, the change in the character of S–O bond and the magnitude of the SO? distortion with increase of K? concentration may be responsible for the unusual variation of spontaneous polarization, P?, with concentration and temperature.

Therefore, from the observation of spectral changes that take place in the low-temperature phases (IV to VI), the phase transitions can be viewed as an order-disorder type with methyl rotation disorder in the high-temperature paraelectric phase. It is also seen that there exists a strong coupling between the deformation of CH? groups and the distortion of the (ZnCl?)²? in the low-temperature phase and could be related to the ferroelectric behaviour in phase III. The present Raman spectroscopic study of SEM·HBr indicates that only minor changes occur in the lattice region of the spectrum and no major changes take place in terms of intensity and linewidth for the various bands due to internal vibrations when the crystal is cooled to the ferroelectric phase. This evidence supports, like in the case of SEM·HCl (12), the conclusions from X-ray diffraction that only minor structural changes are associated with the room-temperature phase transitions. However, the various experimental results obtained by different authors can be discussed within the framework of Landau’s phenomenological theory of phase transitions. Lavanyuk et al. (13) have pointed out that there can be pronounced dielectric anomalies even in the non-ferroelectric transitions due to two other transition parameters where the polarization is not the order parameter. Therefore, the non-observation of hysteresis loops, combined with the fact that there are thermal anomalies but disagreement in the nature of dielectric anomalies reported in this crystal, suggest that these two compounds SEM·HCl and SEM·HBr exhibit a non-ferroelectric, second-order phase transition and the nature and the magnitude of the peaks in the dielectric susceptibilities near the phase transitions may depend on the external electric field applied, and this could be the reason for the differences noticed by the various investigators.