Size and temperature dependent cationic redistribution in NiFe₂O₄(SiO₂) nanocomposites: positron annihilation and Mössbauer studies

Abstract

The size reduction to very small nanoparticles has earlier revealed the transformation of NiFe2O4 from its known inverse spinel structure to a normal phase, preceded and probably forced by an abrupt lattice contraction. Since such lattice contractions are also likely to occur due to lowering of temperature, the defects-related properties of NiFe2O4 nanoparticles at low temperatures (300–20 K) were studied using positron lifetime and Doppler broadened line shape parameter measurements. A change in the predominant positron trapping sites from inherent structural vacancies within the spinel structure to the large microvoid-like cavities at the intersection of the grain interfaces during pelletization of the powdered samples has been identified from the values of the measured positron lifetimes. Significant, and at the same time anomalous, changes have been observed to take place around 90 K owing to an inversion of the normal spinel structure. At lower temperatures, cationic redistribution will take place due to the strong preference of Ni2+ ions towards the octahedral sites and of Fe3+ ions towards the tetrahedral sites, thereby attaining a state of lower energy. The cationic redistribution due to the change in temperature is verified through Mössbauer spectroscopic studies as well. The results are encouraging from the point of investigating identical structural transformations brought in by grain size reduction and/or lowering of temperatures in similar materials.