COMPARED RAMAN-STUDY OF THE PHASE-TRANSITIONS IN K2ZNCL4 AND RB2ZNCL4, RB2ZNBR4, K2SEO4

The Raman spectra of K//2ZnCl//4 single crystals were measured over the temperature range covering the successive phase transitions at T//i equals 553 K, T//L equals 403 K down to 10 K. An amplitude mode is detected below T//i. Evidence for a new phase below T//O 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 K//2ZnCl//4 and Rb//2ZnBr//4 at T//O equals 175 K and 107 K respectively. It is shown that group theoretical considerations applied to experimental Raman data of Rb//2ZnCl//4 are not sufficient to assign the space group of this low temperature phase in K//2ZnCl//4, Rb//2ZnCl//4 and Rb//2ZnBr//4. The mechanism of the transition from the paraelectric phase to the incommensurate phase is discussed for the four compounds K//2SeO//4, K//2ZnCl//4, Rb//2ZnCl//4 and Rb//2ZnBr//4 on the basis of Raman and neutron data obtained in the vicinity of T//i; the possibility of a crossover between a displacive regime and an order disorder regime is emphasized.