Stabilization of α-BiFeO₃ structure by Sr²⁺ and its effect on multiferroic properties

We report a study on the effect of the substitution of Bi³⁺ by Sr²⁺ on the stabilization of R3c structure of Bi_1−xSr_xFeO₃ (0 ≤ x ≤ 0.3, Δx = 0.05), and its effect in the magnetic and dielectric behavior. Stoichiometric mixtures of Bi₂O₃, Fe₂O₃ and SrO were mixed and milled for 5 h using a ball to powder weight ratio of 10:1 by high-energy ball milling. The obtained powder were pressed at 900 MPa to obtain cylindrical pellets and sintered at 800 °C for 2 h. X-ray diffraction and Rietveld refinement were used to evaluate the effect of Sr²⁺ on the crystal structure. In addition, vibrating sample magnetometry (VSM) and dielectric tests were used for describing the multiferroic behavior. The results show that Sr-doped BiFeO₃ particles present rhombohedral structure (R3c) characteristic of α-BiFeO₃ when the doping is below 0.10 mol of Sr. Additionally, a gradual decrease in the amount of secondary phases with the increase of the amount of strontium is observed. For doping concentration higher than 0.15 mol of Sr, a phase transition to an orthorhombic symmetry (β-BiFeO₃, Pbnm) is detected. Besides, changes in relative intensities of reflection peaks planes (110) and (104) are associated with the phase transformations and with the magnetic and dielectric behavior. The α-BiFeO₃ phase show antiferromagnetic behavior and high values of dielectric permittivity, whereas the β-BiFeO₃ phase show a ferromagnetic behavior and low dielectric permittivity.