Understanding dopant site preferences in doped iron oxide nanoparticles : does a relaxed unit cell in nanoparticle alter the site preference within the spinel structure?

Abstract

The dopant behavior of spinels has been investigated for over half a century and yet new insight into this class of materials is still being made today. The dominating question has been "Into which site in the spinel structure does the dopant substitute?". In this work, we will explore this question for the nanoparticle regime. Through this work the potential for a relaxation of the normal strains that can arise in a bulk crystal structure is demonstrated in nanoparticles. The hypothesis that this relaxation can lead to unconventional dopant site preferences for dopants in an iron oxide spinel structure is demonstrated. Nanoparticles ranging from 6 nm to 15 nm in diameter have been synthesized with vanadium, manganese, zinc and gallium doped into the iron oxide spinels. The size and structure of the nanoparticles was investigated with transmission electron microscope and X-ray scattering pair distribution functions. The dopant's valence state was investigated with X-ray absorption spectroscopy and the coordination and magnetic properties of the materials were investigated with X-ray magnetic circular dichroism. Alternating current magnetic susceptibility was used to determine the degree of interaction between the particles, and in the case of non-interacting particles, anisotropy energies were extracted. In this study the dopant atoms were found to behave similarly to their bulk counterparts, with the important exception of manganese and vanadium. Manganese doped iron-oxide nanoparticles show clear evidence of crystalline relaxation. Vanadium substituted into the preferred tetrahedral site in the nanoparticle form, unlike the bulk behavior. Both observations are attributed to the accommodating relaxation found in nanoparticles.