Effect of oxygen vacancies on the microstructure and multiferroic behavior of Bi4.25La0.75Fe0.5Co0.5Ti3O15 ceramics

In order to reveal the solid relationship between oxygen vacancies and multiferroic properties, polycrystalline Bi4.25La0.75Fe0.5Co0.5Ti3O15 (BLFCT) ceramics were sintered in argon (BLFCT-Ar), air (BLFCT-air) and oxygen (BLFCT-O2) by conventional solid state reaction, respectively. Their microstructures, ferroelectric, magnetic properties and valence states of magnetic ions were investigated and compared. X-ray diffraction patterns confirmed a single phase crystal structure in all samples. The lattice constants were calculated and the minor variation of the lattice constants is attributed to the different oxygen vacancy concentration. Furthermore, different oxygen vacancy concentration may be responsible for the different values of RT-recorded remanant magnetization (2Mr), remanent polarization (2Pr) as well as magnetic phase transition temperatures (TCM). The magnetic response of sample sintered in argon (2Mr =0.52 emu/g, TCM=392 K) is significantly superior to that of the others, while the sample sintered in oxygen exhibits a better remnant polarization (2Pr =11.6 µC/cm2) at an applied electric field of 160 kV/cm. The BLFCT-Ar sample was then annealed in oxygen to further justified the dependence of 2Pr and 2Mr on oxygen vacancies. Finally, outcome of the XPS measurement manifested the ratios of Fe2+/Fe3+ and Co2+/Co3+, and reconfirmed the different oxygen vacancy concentration in three samples.