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A fine structure of unknown origin, well resolved at low temperature, has been observed on the low frequency side of the strongest internal mode, at 842 cm-1, in the Raman spectrum of K2SeO4. It consists of three sidebands, monotonically decreasing in intensity, at about 10 cm-1 intervals. The behavior of this fine structure with varying temperature in pure and doped crystals was found to be essentially the same. This suggests that the defect-induced scattering (reported previously) arises from intrinsic defects. Uniaxial stress produces marked softening of the external modes.

The effects of pressure on the dielectric properties and phase transition in K2SeO4 were investigated. The incommensurate-to-commensurate ferroelectric transition vanishes at 0.73 GPa, and the paraelectric-to-incommensurate transition is expected to vanish at ∼ 1.13 GPa. These large pressure effects can be understood in terms of the delicate balance between Coulomb and short-range interactions. The results show the increased influence of quantum effects on approaching the displacive limit (i.e. Tc = O K), and they confirm the existence of a γ = 2 regime in the temperature dependence of the susceptibility, as predicted by recent theories.

Raman spectra of the internal modes of K2SeO4 in the 300-450 cm-1 and 800-950 cm-1 ranges, contain the same nine lines for each of the four crystallographically-allowed irreducible representations of the Raman tensor and except for thermal broadening, remain essentially the same from 20 degK to above 400 degK. From symmetry, 6A1g, 6B1g, 3B2g and 3B3g lines are expected in the 135K

The temperature dependence of the dielectric function of K2ZnCl4deduced from infrared reflection spectra is reported over the PE, INC and FE phases. Whereas differences in the transition mechanisms at Tlbetween K2SeO4and K2ZnCl4and Rb2ZnCl4are recognized, the origin of the dielectric constant anomalies at Tland TCare discussed within the context of a same model in particular the lock-in transition is understood as an instability of the lattice against correlated discommensuration sliding fluctuations. In the INC phase, the spectra of the three compounds show the increase of additional modes. Their number is compatible with incipient tripling of the PE unit cell along the pseudo-hexagonal axis. No additional mode is observed in the FE phase and no significant change of regime occurs around TCin the temperature dependence of mode intensities.