Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 107, Issue 7, Pages 2787-2792Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0910170107
Keywords
crystal structure; disorder; nano-sized ferrimagnets; soil formation; strain
Categories
Funding
- National Science Foundation [CHE-0431425, EF-0830093]
- U.S. Department of Energy (DOE) [DE-AC02-06CH11357]
- Spain's Ministry of Science and Innovation
- European Regional Development [MAT2008-01489, AGL2006-C03-02]
- Chinese Academy of Sciences
- National Nature Science Foundation of China [40821091]
- Div Of Biological Infrastructure
- Direct For Biological Sciences [830093] Funding Source: National Science Foundation
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The natural nanomineral ferrihydrite is an important component of many environmental and soil systems and has been implicated as the inorganic core of ferritin in biological systems. Knowledge of its basic structure, composition, and extent of structural disorder is essential for understanding its reactivity, stability, and magnetic behavior, as well as changes in these properties during aging. Here we investigate compositional, structural, and magnetic changes that occur upon aging of 2-line ferrihydrite in the presence of adsorbed citrate at elevated temperature. Whereas aging under these conditions ultimately results in the formation of hematite, analysis of the atomic pair distribution function and complementary physicochemical and magnetic data indicate formation of an intermediate ferrihydrite phase of larger particle size with few defects, more structural relaxation and electron spin ordering, and pronounced ferrimagnetism relative to its disordered ferrihydrite precursor. Our results represent an important conceptual advance in understanding the nature of structural disorder in ferrihydrite and its relation to the magnetic structure and also serve to validate a controversial, recently proposed structural model for this phase. In addition, the pathway we identify for forming ferrimagnetic ferrihydrite potentially explains the magnetic enhancement that typically precedes formation of hematite in aerobic soil and weathering environments. Such magnetic enhancement has been attributed to the formation of poorly understood, nano-sized ferrimagnets from a ferrihydrite precursor. Whereas elevated temperatures drive the transformation on timescales feasible for laboratory studies, our results also suggest that ferrimagnetic ferrihydrite could form naturally at ambient temperature given sufficient time.
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