Doping level effects in Gd/Cr co-doped Bi₃TiNbO₉ Aurivillius-type ceramics with improved electrical properties

In this work, different amount of Cr₂O₃ (x = 0–0.3 wt%) as dopant were doped into the Aurivillius-type compound Bi_2.8Gd_0.2TiNbO₉ (BGTN), such a kind of Gd/Cr co-doped Bi₃TiNbO₉ ceramics with improved electrical properties were synthesized by the convenient solid-state reaction route. The substitution of Cr³⁺ for Ti⁴⁺ at B-site induced the lattice distortion of pseduo-perovskite layer. Fewer Cr₂O₃ dopant (x < 0.2) resulted in the grain refinement of ceramics. After Cr₂O₃ was added into BGTN, T_C decreased to the vicinity of 908 ℃. Below T_C, the relaxed dielectric response resulted from charge carriers hopping induced another board dielectric permittivity peak, whose starting temperature shifts toward lower side gradually with increase of x. The values of E_a^con calculated from the Arrhenius relationship between conductivity and temperature indicated the intrinsic conduction at high temperature is dominated by the long-range migration of doubly ionized oxygen vacancies. Moderate Cr₂O₃ dopant (x = 0.1–0.25) are conducive to the enhancement of piezoelectric property and thermal stability. The sample with x = 0.2 achieved both a high T_C~903 ℃ and a high d₃₃~18 pC/N at the same time. Also, its d₃₃ can retain 80% of the initial value after the sample was annealed at 800 ℃ for 4 h.