Alkaline earth cation substitution effects in LaFeO3 orthoferrite nanocrystals studied by positron annihilation

Alkaline earth (Ae = Ca, Sr and Ba) cation substituted La1-xAexFeO3 (x = 0.1, 0.2, …, 0.5) nanocrystalline samples were prepared through the normal chemical route and were characterized by X-ray diffraction. Optical absorption studies showed an increase of the band gap with increasing x in the case of Ca and Sr but a final decrease again in Ba substitution. Positron lifetime measurements indicated decrease in annihilation at the nanocrystallite surfaces and increase in the additional defects created within the nanocrystallites. Coincidence Doppler broadened spectra showed prominent peaks at pL (10−3 m0c) = 9.8 due to the annihilation of positrons with the O2-2p electrons, which implied that the predominant defects which trapped positrons were vacancies of cations and their complexes. In the Ba-substituted samples, the annihilations with the Ba2+ 4d electrons reduced the intensity of O2-2p electron annihilation with positrons.