Study of Ce3+-substitution induced defects restructuring in BiFeO3 by positron annihilation and other supportive methods

Abstract

The results of Ce3+-substitution in place of Bi3+ in BiFeO3 nanocrystallites are reported in this work. Although Bi1-xCexFeO3 (x = 0, 0.025, …, 0.175) was formed with distinct crystallographic planes of X-ray diffraction, no structural transformation was observed unlike in the case of other lanthanide ion substitution of Bi3+ ions as reported earlier. The nanocrystallite sizes and the lattice parameters decreased during the increase of substitution. Transmission electron microscopic images also revealed the nanostructure formation and the decrease of the particle sizes. Energy dispersive X-ray analysis revealed appreciable incorporation of Ce into the BiFeO3 lattice. The measured band gap energies decreased owing to the creation of acceptor levels by Ce3+ ions. Distinct peaks in the X-ray photoelectron spectra at the expected binding energies indicated the presence of the two oxidation states Ce3+ and Ce4+. Positron annihilation studies proved the formation of interfacial defects with increase in substitution. Majority of the positrons diffused to the surfaces of the nanocrystallites and the mean lifetime of positrons increased. Coincidence Doppler broadened spectra, however, indicated positron trapping in the interfacial defects of the oxides of Ce and Bi overriding the annihilations of positrons at the surfaces in the range x = 0.075 to 0.125.