Microstructure and properties of Co-, Ni-, Zn-, Nb- and W-modified multiferroic BiFeO3 ceramics

BiFeO3 polycrystalline ceramics were prepared by the mixed oxide route and a chemical route, using additions of Co, ZnO, NiO, Nb2O5 and WO3. The powders were calcined at 700 °C and then pressed and sintered at 800–880 °C for 4 h. High density products up to 96% theoretical were obtained by the use of CoO, ZnO or NiO additions. X-ray diffraction, SEM and TEM confirmed the formation of the primary BiFeO3 and a spinel secondary phase (CoFe2O4, ZnFe2O4 or NiFe2O4 depending on additive). Minor parasitic phases Bi2Fe4O9 and Bi25FeO39 reduced in the presence of CoO, ZnO or NiO. Additions of Nb2O5 and WO3 did not give rise to any grain boundary phases but dissolved in BiFeO3 lattice. HRTEM revealed the presence of domain structures with stripe configurations having widths of typically 200 nm. In samples prepared with additives the activation energy for conduction was in the range 0.78–0.95 eV compared to 0.72 eV in the undoped specimens. In co-doped specimens (Co/Nb or Co/W) the room temperature relative permittivity was ∼110 and the high frequency dielectric loss peaks were suppressed. Undoped ceramics were antiferromagnetic but samples prepared with Co or Ni additions were ferromagnetic; for 1% CoO addition the remanent magnetization (MR) values were 1.08 and 0.35 emu/g at temperatures of 5 and 300 K, respectively.