Structural transitions and multiferroic properties of high Ni-doped BiFeO₃

Nickel doped bismuth ferrite powders, BiFe_1−x Ni_xO₃ (0 ≤ x ≤ 0.5), were synthesized by high-energy ball milling followed by an annealing at 700 °C. A detailed study about the substitution of Fe³⁺ by Ni²⁺ on the crystal structure and multiferroic properties is presented. The X-ray diffraction patterns reveal the formation of rhombohedral structure with small amounts of Bi₂Fe₄O₉ as a secondary phase for x < 0.1. Also it is inferred the stabilization of a Bi₂₅FeO₄₀, sillenite phase, as the amount of Ni²⁺ substitution increases, reaching up 95.23% of sillenite for x = 0.5. The magnetic behavior indicates the frustration of the G-antiferromagnetic order typical of the un-doped BiFeO₃, caused by the presence of small amounts of Ni²⁺ (x < 0.1) on the structure. The DC conductivity exhibited a little increment with increasing Ni content (up to x = 0.1). Although the conductivity increases, for nickel concentrations of 0.2–0.5, the bismuth ferrites doped with nickel retain their property of being an electrical insulating material. Behavior modifications of electrical conductivity, permittivity and dielectric loss versus frequency are related with crystal structure transformations, when nickel concentration is increased.