Raman Studies on Structure Transitions of Layered Ferroelectrics Ba_2+xBi_4-xTi_5-xNb_xO₁₈

Raman spectra of layered ferroelectrics Ba_2+xBi_4-xTi_5-xNb_xO₁₈ (0 ≤ x ≤ 2) at room temperature in the range 20-1000cm^-1 were investigated. Vibration modes were assigned with the use of internal mode approach. Two doping-induced structure phase transitions were observed at about x = 0.50 and x = 1.25, respectively. Raman spectra variations were analyzed according to the selection rules of BO₆ octahedra in various symmetry. Polycrystalline samples were synthesized by solid state reaction method.