A study of Cr3+-substitution induced defects restructuring in BiFeO3 by positron annihilation and other supportive methods

The effects of Cr3+ ion substitution in place of Fe3+ ions in BiFe1-xCrxO3 nanocrystallites are reported in this work. Due to the smaller radii of Cr3+ ions, the nanocrystallite sizes and lattice parameters decreased during the substitution. The band gap energy decreased due to the increased conduction band edge tailing. The ionic concentrations, estimated from the energy-dispersive analysis of X-rays, indicated that there is only partial replacement of Fe3+ ions and a fraction of the Cr3+ ions either occupied the pre-existed Bi3+ ion vacancies or replaced some of the Bi3+ ions. Positron annihilation studies proved that, with increasing doping by Cr3+ ions, Bi3+- and O2− ion vacancies were generated. The diffusion of positrons to the surfaces of the nanocrystallites, however, continued to enhance the mean positron lifetimes. Coincidence Doppler broadened spectra also hinted at the generation of divacancies of Bi3+ and O2− ions. Dielectric measurements showed that the charge excess of the VBi3--VO2+ divacancies got balanced by a reduction of the Fe3+ ions to Fe2+ states. Magneto-electric coupling enhanced due to the quenching of the cycloidal-spiral spin structure and the increased Fe–O–Fe/Cr bond-bending. No structural transformation was observed unlike in the case of lanthanide ion substitution of Bi3+ ions.