Doping-Induced Structure Phase Transition of Layered Ferroelectrics Sr_2+xBi_4-xTi_5-xNb_xO₁₈

Raman spectra of layered ferroelectrics Sr_2+xBi_4-xTi_5-xNb_xO₁₈ (0 ≤ x ≤ 2) and Sr_2+xBi_4-xTi_5-xNb_xO₁₈ at room temperature in the range of 20-1000cm^-1 were investigated. Vibration modes were assigned with the internal mode approach. Two doping-induced structure phase transitions were observed at about x = 0.5 and 1.25 for Sr_2+xBi_4-xTi_5-xNb_xO₁₈, separately.