![]() ![]() The established effect of depletion of the nanoparticle surface manifests itself in the low nanoparticle fractal dimension ( ≈2.82) determined by the small-angle X-ray scattering technique. However, the particle itself remains chemically stable. Due to such a feature of the ferrihydrite surface formation, the surface enthalpy is much lower in comparison with other iron oxides and hydroxides. As it was mentioned in Ref. , the ferrihydrite particle surface is depleted in iron and rich in OH-groups, that strongly differs from the structure of the particle core. As was found in Ref. , the structure of ferrihydrite polyhedra near the surface, which is depleted in iron and rich in OH-groups, strongly differs from the structure of the particle core. A nanosized ferrihydrite particle can be presented by a model with different surface (shell) and internal (core) positions of iron atoms. Study of aggregation of ferrihydrite nanocrystals showed that the formation of aggregates cannot be described by conventional growth models. The obtained X-ray data made it possible not only to refine the crystal structure and chemical formula of ferrihydrite, but also to determine the degree of its atomic disorder. The crystal structure of ferrihydrite and its chemically synthesized aggregates was studied in Refs. . The conditions for the formation of ferrihydrite significantly affect the physical properties of its nanoparticles, concerning particularly its crystallinity and interactions between individual particles, , ]. It is noteworthy that the organic shell makes the magnetic interparticle interactions negligible. In the most well-studied horse spleen ferritin, the ordered ferrihydrite core is located inside the protein shell with outer and inner diameters of 12 and 5–8 nm, respectively. In living organisms, ferrihydrite forms in the core of a ferritin complex consisting of a protein shell capsule with the ordered hydrated iron oxide core. This mineral is involved in the vital activity of higher animals, including humans. Then, the size of crystallized biogenic ferrihydrite particles is no more than ≈10 nm, ,, ,, ,, ,, ,, , ]. Ferrihydrite can appears also in the bacterial life cycle. This mineral is of crucial importance for microorganisms’ life. A huge surface makes ferrihydrite a good sorbent of heavy metals. The discovery of ferrihydrite on Mars indirectly confirmed the presence of water on this planet. This mineral with the nominal formula Fe 2 O 3 × n ⋅ H 2 O precipitates as a product of nucleation of hydrolyzed ferric ions with increasing pH. The well-crystallized core is formed only for nanoparticles larger than ≈2 nm, whereas smaller particles consist entirely of a matter with a lower density of iron atoms.įerrihydrite plays a decisive role in the circulation of iron and controls the iron concentration in aqueous systems. We found that the size of the dense core depends on the particle size. According to the Mössbauer data, we propose a core-shell structural model of the biogenic ferrihydrite particles. We established that the exceptional magnetic anisotropy of ferrihydrite ( K V = 1.2 ⋅ 10 5 erg/cm 3 and K S = 0.1 erg/cm 2) is reached because of the highly developed ferrihydrite nanoparticles’ surface. Based on the Mössbauer data, we identified the superparamagnetic blocking temperature at the temperature of 30 K for the largest ferryhidrite particles. The features caused by the surface effects and the inhomogeneous structure of ferrihydrite have been examined in the temperature range of 4–300 K using Mössbauer spectroscopy and magnetometry. In this study, we investigated the biogenic ferrihydrite nanoparticles with the narrow size distribution and an average diameter of ≈ 2 nm obtained by the bacteria life cycle. The uncompensated magnetic moment of the ferrihydrite caused by the antiferromagnetic ordering of the magnetic moments of iron atoms and leads to the magnetic properties very similar to those of ferro- and ferrimagnetic nanoparticles. Ferrihydrite is a low-crystalline nanoscale matter. ![]()
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