Microstructure characterization of mechanosynthesized nanocrystalline NiFe2O4 by Rietveld's analysis

Nanocrystalline nickel ferrite (NiFe2O4) is synthesized at room temperature by high-energy ball milling the stoichiometric mixture of (1:1 mol%) of NiO and α-Fe2O3 powders. The structural and microstructural evolution of NiFe2O4 caused by milling is investigated by X-ray powder diffraction. The relative phase abundance, particle size, r.m.s. strain, lattice parameter changes of different phases have been estimated employing Rietveld structure refinement analysis of X-ray powder diffraction data. Particle size and content (wt%) of both NiO and α-Fe2O3 phases reduce rapidly with increasing milling time and a significant amount of nanocrystalline NiFe2O4 is formed within 1 h of ball milling. Particle sizes of all the phases reduce to ∼10 nm within 5 h of milling and remain almost unchanged with increasing milling time up to 20 h. Lattice parameter of cubic NiO decreases linearly with increasing milling time, following the Vegard's law of solid-solution alloy. A continuous decrease in lattice parameter of cubic NiFe2O4 phase clearly suggests that smaller Ni atoms have occupied some of the vacant oxygen sites of ferrite lattice. Cation distribution both in octahedral and tetrahedral sites changes continuously with milling time and the normal spinel lattice formed at early stage of milling, transforms to inverse spinel lattice in the course of milling. High-resolution transmission electron microscope (HRTEM) micrographs of 11 h milled sample corroborates the findings of X-ray profile analysis.