PRECURSOR EFFECTS AND SUCCESSIVE PHASE-TRANSITIONS IN A DILUTE CRYSTAL OF KTN

The light depolarization and Raman scattering spectrum have been measu red as a function of the temperature in a dilute crystal of potassium tantalate niobate (KTN) with 2% Nb. The careful analysis of the Raman data versus temperature clearly reveals for the first time the sequenc e of two phase transitions below the cubic phase around 40 K and 32 K, connected to the dipolar ordering. The occurrence, well above 40 K, o f Raman lines unexpected from the symmetry and attributed to the phono n density of states, as well as the quasi-elastic scattering, is inter preted in terms of the precursor phenomena and dynamic clusters in the cubic paraelectric phase.