Synthesis and electrical properties of Sr- and Nb-cosubstituted Bi_4-xSr_xTi_3-xNb_xO₁₂ polycrystalline thin films

Lead-free ferroelectric Bi₄Ti₃O₁₂ (BIT) thin films were modified by the cosubstitution of Sr²⁺ for Bi³⁺ and of Nb⁵⁺ for Ti⁴⁺ by spin-coating and decomposition of chemical solutions of metal-alkoxide materials (the nominal compositions of Bi_4-xSr_xTi_3-xNb_xO₁₂ (BSTN) are x = 0.5, 1.0, 1.5). Single-phase thin films with a BIT-type structure were crystallized above 550°C. At room temperature, the ferroelectric and dielectric properties were found to be P_r = 10 μC/cm², E_c = 100 kV/cm, ε_r = 300, and tan δ < 5% for x = 0.5 after annealing at 650°C. The films with x = 1.0 and 1.5 did not exhibit ferroelectric hysteresis behavior because of the decrease in Curie temperature (T_c) below room temperature. The x dependence of of T_c was studied by considering the soft mode behavior in Raman scattering spectra and the T_c values were 400, -25, and -220°C for x = 0.5, 1.0, and 1.5, respectively. The cosubstitution by Sr²⁺ and Nb⁵⁺ is effective for reducing ferroelectric interaction between electrical dipoles leading to a large shift in T_c. Because of its high solubility of Sr²⁺ and Nb⁵⁺ and efficiency for shifting T_c, the BSTN system may find a novel application as a dielectric material rather than as a ferroelectric material.