ORDERED FERRIMAGNETIC FORM OF FERRIHYDRITE REVEALS LINKS BETWEEN STRUCTURE, COMPOSITION AND MAGNETISM

Key to understanding the reactivity and stability of most nanomaterials is the development of rigorous relationships between crystal structure and fundamental physiochemical properties such as composition and magnetic behavior. This is particularly challenging for materials with extremely small particle sizes (< 10 nm) and complex structural disorder. Ferrihydrite is a quintessential example. This hydrated ferric oxyhydroxide nanomineral is an important component of many environmental and soil systems, acting as a strong sorbent for heavy metal and metalloid contaminant ions. Additionally, it has been implicated as the inorganic iron core of ferritin in biological systems.

Despite its widespread natural and synthetic occurrences, the basic crystal structure of ferrihydrite and its physical properties (e.g,. magnetism, density, and composition) remain uncertain and controversial. These uncertainties are in large part because ferrihydrite has no known synthetic or natural crystalline counterpart that can be used to establish its basic structure and physiochemical properties. Here we show for the first time that the synthesis and aging of ferrihydrite in the presence of citrate produces an ordered, nanocrystalline form, which is a major breakthrough for resolving the long-standing controversies concerning its structure-property relationships. We show that the antiferromagnetic-like behavior of disordered ferrihydrite, which becomes ferrimagnetic during aging with citrate, is due to structural ordering, compositional changes, and particle growth. We use synchrotron-based x-ray scattering methods, complemented by physical, chemical, and magnetic data, to unravel the compositional and structural changes that lead to a dominantly ferrimagnetic behavior not previously associated with ferrihydrite. Our results provide the first direct links between crystal structure and magnetism in ferrihydrite and represent a critical advance in our understanding of its structural disorder. These findings serve to validate a controversial structural model for this phase (Michel, F. M. et al.. Science 316, 1726-1729 (2007)) and also provide a new understanding of the density and composition of ferrihydrite.