Ferroelectricity in Bi₄Ti₃O₁₂ and its solid solutions

Abstract

On the basis of dielectric studies on polycrystalline specimens, Bi₄Ti₃O₁₂ has been established as a ferroelectric with a Curie temperature of 675°C. The symmetry is orthorhombic with a = 5.411 A, c = 32.83 A, and b/a = 1.007 at 25°C. A high-temperature x-ray study revealed a symmetry change to tetragonal at 675°C. The polar axis is probably the orthorhombic b axis. Bi₄Ti₃O₁₂ is a member of oxides with the general formula (Bi₂O₂)²⁺(Me_x-1R_x O_3x+1)^2-. According to Aurivillius, the crystal structure of Bi₄Ti₃O₁₂ comprises a stacking of Bi₂O₂ and perovskite-like Bi₂Ti₃O₁₀ layers along the pseudotetragonal c axis. Multiple ion substitutions of (Bi³⁺Ti⁴⁺) in (Bi₂O₂)²⁺(Bi₂Ti₃ O₁₀)^2- by (Me²⁺Nb⁵⁺) where Me²⁺=Ba, Pb, or Sr lead to a steep decrease of the Curie temperature.