Compare raman study of the phase transitions in K2ZnCl4 and Rb2ZnCl4, Rb2ZnBr4, K2SeO4

The Raman spectra of K2ZnCl4 single crystals have been measured over the temperature range covering the successive phase transitions at Ti = 553 K, TL = 403 K down to 10 K. An amplitude mode is detected below T i. Evidence for a new phase below T0 is given by the observation of a soft phonon mode. The measured temperature dependence of the birefringence clearly confirms the existence of this low temperature phase in K2ZnCl 4 and Rb2ZnBr4 at T0 = 175 K and 107 K respectively. It is shown that group theoretical considerations applied to experimental Raman data of Rb2ZnCl4 are not sufficient to assign the space group of this low temperature phase in K2ZnCl4, Rb2ZnCl 4 and Rb2ZnBr4. The mechanism of the transition from the paraelectric phase to the incommensurate phase is discussed for the four compounds K2SeO4, K2ZnCl4, Rb2ZnCl4 and Rb2ZnBr4 on the basis of Raman and neutron data obtained in the vicinity of Ti ; the possibility of a crossover between a displacive regime and an order disorder regime is emphasized.