Studies of structural, electrical and ferroelectric properties of rare-earth substituted bismuth titanate-films.

Abstract

In the present research, Bi₄Ti₃O₁₂ (BTO) and its rare earth (La, Nd, Sm, Gd)-substituted derivative thin films were synthesized using a sol-gel method and were systematically investigated. Films were deposited by spin coating on Pt (Pt/Bi₄Ti₃O₁₂) substrates followed by their annealing at 700°C in air. According to the x-ray diffraction analysis of Bi_4-xM_xTi₃O₁₂ (M = La, Nd, Sm, Gd) patterns showed polycrystalline materials with preferential (117) orientation.Microstructure by SEM and X-ray studies shows that the crystallite sizes decrease as the rare earth (La, Nd, Sm, Gd) content increase.Raman spectroscopy analysis suggest that the Rare earth ions (La³⁺ Nd⁺³, Sm⁺³, Gd⁺³) are incorporated into the Bi site (A site). On the other hand, Bi_3.44La_0.56Ti₃O₁₂, Bi_3.44Nd_0.56Ti₃O₁₂, Bi_3.30Sm_0.70Ti₃O₁₂ and Bi_3.70Gd_0.30Ti₃O₁₂ thin films capacitor showed significantly improved values of the remanent polarization P_r = 47.59, 70, 29 and 27.5 μC/cm² and enhanced fatigue-free behavior respectively. Their dielectric constants and losses were 535, 0.09, 434, 0.07, 344, 0.039 and 333, 0.039 respectively at the frequency 1MHz. At lower field, the leakage current is dominated by ohmic behavior in all thin films. Bi_3.70Gd_0.30Ti₃O₁₂ showed the lowest leakage current 10^-8 A/cm² under low fields. Bi_3.15Gd_0.85Ti₃O₁₂ powder was tested using SQUID, which showed super-paramagnetic at room temperature and magnetic behavior at 5 K with magnetic coercitivity (Hc = 29 Oe).