Reversal in the Lattice Contraction of α‑Fe₂O₃ Nanoparticles

We report the discovery of size-dependent lattice contractions and elongations in the nanoscale hematite (α-Fe₂O₃) structure revealed by high-resolution X-ray diffraction analysis and spectroscopic measurements. The observed lattice modification effects are classified into two different regions with the crystallite size (CS) with a threshold anomaly at ≈30 nm. In addition to the commonly observed lattice expansion in ionic solids, we report the discovery of lattice contraction at the nanoscale level (<30 nm CS). The consequences of anomalous structural behavior with the CS are reflected in the electronic and vibrational properties of the hematite structure. The characteristic behavior in structural and electronic properties of the hematite structure is closely linked with changes in the bonding character, which shows strong dependence on the CS. We suggest that the lattice expansion is caused by weaker hybridization of e_g states, whereas at the nanoscale regime the increased level of Fe 4sp–O 2p hybridization leads to less oxygen 2p to hybridize with the 3d-states, implying an increased level of covalency and reduction in the unit cell parameters. Furthermore, the change in the bonding characteristics leads to the enhanced polaronic conductivity of 4 orders of magnitude at the nanoscale level, which is highly beneficial for the unique structural advantage of the iron oxide and its derived compounds