Local Structure of Core–Shell MnFe₂O_4+δ-Based Nanocrystals: Cation Distribution and Valence States of Manganese Ions

Abstract

We investigate the local structure of nanoparticles based on a manganese ferrite core surrounded or not by a maghemite layer obtained after hydrothermal surface treatment. Results of X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) measurements are crossed with those of infield Mossbauer spectroscopy and X-ray absorption spectroscopy (XANES/EXAFS) to study the valence state of Mn ions and the cation distribution at interstitial sites of the core−shell nanoparticle structure. Linear combination fitting of XANES data clearly indicates the existence of mixed valence states of Mn cations in the Mn ferrite phase. As a direct consequence, it induces nonequilibrium cation distributions in the nanoparticle core with the presence of a large amount of Mn cations at octahedral sites. The quantitative results of the inversion degree given by NPD, Mossbauer spectroscopy measurements, and EXAFS are in good accordance. It is also shown that both the proportions of each oxidation degree of Mn ions and their location at tetrahedral or octahedral sites of the spinel nanocrystal core can be modified by increasing the duration of the surface treatment. a χ M is the molar fraction of manganese ions obtained by ICP and AAS techniques, ⟨a⟩ is the average lattice parameter deduced from Bragg's law, ϕ c / ϕ is the volume fraction of the core, and t sh is the thickness of the surface layer. The particle sizes (D XR ) were obtained by Scherrer's equation.

SAXS Analysis of Magnetic Field Influence on Magnetic Nanoparticle Clusters

“…The calculation of the molar fraction is given by, χ m = [Mn]/([Mn] + [Fe]). Studies correlated to this mechanism of the core-shell model were reported in the works of Gomes et al [34], Martins et al [23], and Pilates et al [26]. Figure 1a presents the TEM picture of the sample and shows that the NPs are roughly spherical.…”

“…An excess of positive charges occurs due to the oxidation of Mn +2 to Mn +3 in the synthesis process, in order to balance exactly the extra positive charge that appears in the formula is inserted a δ parameter of 0.48 to adjust the electronegativity. In recent works by Martins et al [23] and Moreira et al [33], the mean valence of manganese in ferrites synthesized by coprecipitation was determined. Several analyzes were performed, including the X-ray Absorption Near Edge Structure (XANES) analysis with the Linear Combination Fitting (LFC) method of Mn 3 O 4 , Mn 2 O 3 and MnO 2 oxides.…”

“…These models were successfully used to characterize a disordered cluster structure [19], a fractal mass of particles [20] as well as to distinguish between individual polydispersed particles and aggregates [21]. This work aims to investigate the local structure of magnetic colloids [22] under external magnetic field application, the colloids in question are composed of magnetic nanoparticles based on manganese ferrite type core-shell MnFe 2 O 4+δ @γ-Fe 2 O 3 [23] and dispersed in aqueous medium. SAXS measurements were performed with external magnetic field applied [24] to determine the dimensions between nanoparticles and clusters [25], for future studies in hyperthermia [26] for biomedical applications.…”

Redox‐Inert Fe3+ Ions in Octahedral Sites of Co‐Fe Spinel Oxides with Enhanced Oxygen Catalytic Activity for Rechargeable Zinc–Air Batteries

“…The Mössbauer spectrum is shown in Figure c, where Co 2 FeO 4 /NCNTs exhibits superparamagnetic behavior. The composite system could be fitted with two symmetric quadrupole doublets with Lorentzian line shapes and isomer shifts and quadrupole splitting of δ(A1)=0.37 mm s −1 , Δ E Q (A1)=0.60 mm s −1 , and δ(A2)=0.46 mm s −1 , Δ E Q (A2)=0.92 mm s −1 at 300 K. The parameters clearly indicate that the two subspectra correspond to iron at M Oct and M Td sites of the Co 2 FeO 4 /NCNTs . The ratio of Fe 3+ Oct to Fe 3+ Td is 2.57 by integration of the A2/A1 components, indicating that oxide is a hybrid spinel (Co 0.72 Fe 0.28 ) Td (Co 1.28 Fe 0.72 ) Oct O 4 .…”

Redox‐Inert Fe3+ Ions in Octahedral Sites of Co‐Fe Spinel Oxides with Enhanced Oxygen Catalytic Activity for Rechargeable Zinc–Air Batteries

“…[17] TheM çssbauer spectrum is shown in Figure 1c,w here Co 2 FeO 4 /NCNTs exhibits superparamagnetic behavior.T he composite system could be fitted with two symmetric quadrupole doublets with Lorentzian line shapes and isomer shifts and quadrupole splitting of d(A1) = 0.37 mm s À1 , DE Q (A1) = 0.60 mm s À1 ,and d(A2) = 0.46 mm s À1 , DE Q (A2) = 0.92 mm s À1 at 300 K. The parameters clearly indicate that the two subspectra correspond to iron at M Oct and M Td sites of the Co 2 FeO 4 /NCNTs. [18] Theratio of Fe 3+ Oct to Fe 3+ Td is 2.57 by integration of the A2/ A1 components,i ndicating that oxide is ah ybrid spinel (Co 0.72 Fe 0.28 ) Td (Co 1.28 Fe 0.72 ) Oct O 4 .T he isomer shift (d), ad escriptor of the valence state of the Mçssbauer absorber atom, indicates that Fe exists mainly in trivalent form with high crystal field stabilization energy,a nd thus can partly replace Co atoms situated on the octahedral 16 b sites of the Co 3…”