Defects characteristic investigation of bismuth ferrite nanocrystallites through positron annihilation and supportive methods

Abstract

Phase pure bismuth ferrite (BFO) nanocrystallites were synthesized by sol-gel method. The variation in the time of calcination at 600 °C is used as the parameter to vary the sizes of the nanocrystallites. X-ray diffraction and electron microscopic studies revealed the formation of BFO nanocrystallites of well-defined sizes and morphology. The elemental stoichiometric compositions of the samples were verified using energy dispersive analysis of X-rays and further by X-ray photoelectron spectroscopy. The UV–visible diffuse reflectance spectra showed strong absorption at the wavelength ∼556 nm (2.23 eV) for BFO nanocrystallites of dimensions 20 nm. For nanocrystallites of sizes smaller than this limit, the band gap energies showed remarkable increase due to the onset of quantum confinement effects. The band gap energies showed increase at larger crystallites sizes as well, indicating defects formation due to oxygen deficiencies. Positron annihilation spectroscopy is used to characterize and monitor the defects. The formation of Bi3+-O2- divacancies significantly altered the positron annihilation characteristics at larger crystallite sizes.