Positron annihilation studies of some anomalous features of NiFe2O4 nanocrystals grown in SiO2 -: art. no. 024115

Nanocrystalline NiFe2O4 particles were synthesized in a SiO2 matrix and were characterized by x-ray diffraction and TEM observations. Positron lifetimes in these samples were measured. The measured positron lifetimes indicated admixtures of contributions from annihilations within the nanoparticles, at the nanoparticle-SiO2 interfaces, and from the SiO2 matrix. The individual contributions were calculated based on the known characteristics of electron-positron annihilation in solids and they were found in remarkable agreement with the known effects expected from grain size reduction and lattice contraction. However, the anomalous rise in positron lifetimes during the reduction of the grain size below 5.6 nm is a deviation from these expected trends and is attributed to the transformation of the inverse spinel structure of the NiFe2O4 to the normal phase, with the tetrahedral (or A) sites being fully occupied by the divalent Ni2+ ions and the Fe3+ ions transferred to the octahedral (or B) sites. The results of Mossbauer spectroscopic studies supported this argument, as the percentage of Fe3+ ions occupying the A sites drastically reduced to zero when the grain size of the samples decreased to 4.8 nm and below.