EFFECT OF DOPED IONS ON SOFT MODE AND PHASE-TRANSITIONS IN BI LAYEREDCOMPOUND

Raman spectra of Bi(2-x)A(x)Ti(4-y)B(y)O(11)(A = Gd, Yb, La, Nd, Sb, F e; B = Te, Zr, Sn) have been systematically investigated. The results show that these doped ions have different effects on the lowest freque ncy phonon mode (soft mode) and the phase transition in Bi2Ti4O11. The phase transitions in Bi(2-x)A(x)Ti(4)O(11) system is directly associa ted with softening of the lowest frequency phonon mode, which results from the changes in the short-range forces. Temperature, pressure and doped ion concentration - induced phase transition is separately corre lated with an unstable displacement of Bi ions with a charge transfer effect in Bi-O-Ti system, The softening and normal behavior observed c an be explained as due to the effects of local distortion on Bi ion di splacement. The non-zero soft mode frequency at the phase transition m ay come from a coupling between the soft mode and an additional phonon mode.