Lattice Vibrations and Incommensurate Phase Transitions in K2SeO4, Rb2ZnCl4 and K2ZnCl4

The temperature dependence of the dielectric function of K2ZnCl4 deduced from infrared reflection spectra is reported over the PE, INC and FE phases. Whereas differences in the transition mechanisms at Tl between K2SeO4 and K2ZnCl4 and Rb2ZnCl4 are recognized, the origin of the dielectric constant anomalies at Tl and TC are 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 TC in the temperature dependence of mode intensities.