Magnetism and Mössbauer study of formation of multi-core γ-Fe2O3 nanoparticles

A systematic investigation of magnetic nanoparticles and the formation of a core-shell structure, consisting of multiple maghemite (γ-Fe2O3) nanoparticles as the core and silica as the shell, has been performed using various techniques. High-resolution transmission electron microscopy clearly shows isolated maghemite nanoparticles with an average diameter of 13?nm and the formation of a core-shell structure. Low temperature Mössbauer spectroscopy reveals the presence of pure maghemite nanoparticles with all vacancies at the B-sites. Isothermal magnetization and zero-field-cooled and field-cooled measurements are used for investigating the magnetic properties of the nanoparticles. The magnetization results are in good accordance with the contents of the magnetic core and the non-magnetic shell. The multiple-core γ-Fe2O3 nanoparticles show similar behavior to isolated particles of the same size.We thank the assistance by Dr. Peter Klavins at the Department of Physics, the University of California Davis, in performing the magnetization measurements. This research was partially supported by the Department of Energy, Office of Nuclear Energy, Nuclear Energy Program, under Grant No. DE-NE000070

Magnetism of Magnetite nanoparticles as determined by Mössbauer Spectroscopy

Fe3O4 [Magnetite] nanoparticles have magnetism that differs greatly from their bulk counterparts. Whereas bulk Fe3O4 is a ferrimagnet, single-domain Fe3O4 nanoparticles have been found to be superparamagnetic. This allows for increased magnetization of the nanoparticles compared to the bulk when in a magnetic field. For most paramagnets, magnetization requires applied fields of a few Tesla at low temperatures. This is achievable through the application of superconducting magnets. In superparamagnets, the high susceptibility of the particles allows magnetization through a Nd-Fe-B permanent magnet at room temperature. This is caused by an increased number of magnetic atoms within the particles, which greatly increases susceptibility of the particles. 57Co [Cobalt-57] Mössbauer Spectroscopy allows the probing of the internal environment of an iron nucleus, which gives insight into the magnetic properties of the Fe3O4 nanoparticles

Mössbauer Spectroscopy of Iron Oxide Nanoparticles: Materials for Biomedical Applications

Nanoparticles of Fe3O4 (magnetite) and gamma-Fe2O3 (maghemite) have been studied by Mössbauer spectroscopy. These nanoparticles have applications as contrast agents for Magnetic Resonance Imaging. At room temperature, they are superparamagnetic and the magnetic hyperfine fields were averaged to zero. The spectra of Fe3O4 were composed of two lines corresponding to two crystal sites, identified from their isomer shifts as Fe3+ on the tetrahedral A site and Fe2.5+ (mixed Fe2+ and Fe3+) on the octahedral B site. The relative intensity of the two lines shows that the samples are almost stoichiometric with the formula Fe3-xO4, where x is less than 0.04 at room temperature. The x that was obtained was compared to those obtained in a magnetic field and at room temperature. The lines are broad and the measurements as a function of temperature were analyzed using Néel’s theory of superparamagnetism to yield values of the relaxation times. The same nanoparticles that have oxidized into gamma-Fe2O3 have been studied for comparison

Magnetic interactions in Fe1-xMxSb2O4, M = Mg, Co, deduced from Mössbauer spectroscopy.

Magnesium- and cobalt- substituted FeSb2O4, of composition Fe1−xMgxSb2O4 (x = 0.25, 0.50, 0.75) and Fe0.25Co0.75Sb2O4 have been examined by 57Fe Mössbauer spectroscopy. The complex spectra recorded from the magnetically ordered materials are interpreted in terms of two models in which the dominant magnetic interactions occur along the rutile-related chains of FeO6 octahedra in the magnetic structure of FeSb2O4. In materials of the type Fe1−xMgxSb2O4, the diamagnetic Mg2+ ions have no magnetic moment and behave as non-magnetic blocks which disrupt the magnetic interactions in the chains along the c-axis forming segments of iron-containing chains separated by Mg2+ ions. In Fe0.25Co0.75Sb2O4 the spectra are composed of components from different configurations of neighbouring Fe2+ and Co2+ ions

The reaction mechanism of SnSb and Sb thin film anodes for Na-ion batteries studied by X-ray diffraction, 119Sn and 121Sb Mossbauer spectroscopies

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Oxygen insertion reactions within the 1-D channels of phases related to FeSb2O4

The structure of the mineral schafarzikite, FeSb2O4, has one-dimensional channels with walls comprising Sb3+ cations; the channels are separated by edge-linked FeO6 octahedra that form infinite chains parallel to the channels. Although this structure provides interest with respect to the magnetic and electrical properties associated with the chains and the possibility of chemistry that could occur within the channels, materials in this structural class have received very little attention. Here we show, for the first time, that heating selected phases in oxygen-rich atmospheres can result in relatively large oxygen uptakes (up to ~2% by mass) at low temperatures (ca 350°C) whilst retaining the parent structure. Using a variety of structural and spectroscopic techniques, it is shown that oxygen is inserted into the channels to provide a structure with potential to show high one-dimensional oxide ion conductivity. This is the first report of oxygen-excess phases derived from this structure. The oxygen insertion is accompanied not only by oxidation of Fe2+ to Fe3+ within the octahedral chains but also Sb3+ to Sb5+ in the channel walls. The formation of a defect cluster comprising one 5-coordinate Sb5+ ion (which is very rare in an oxide environment), two interstitial O2- ions and two 4-coordinate Sb3+ ions is suggested and is consistent with all experimental observations. To the best of our knowledge, this is the first example of an oxidation process where the local energetics of the product dictate that simultaneous oxidation of two different cations must occur. This reaction, together with the wide range of cation substitutions that are possible on the transition metal sites, presents opportunities to explore the schafarzikite structure more extensively for a range of catalytic and electrocatalytic applications. The neutron diffraction data are contained in two folders corresponding to the instruments: 1. HRPT with one data file and the instrument parameter file. 2. D20 with one instrument parameter file and five sub-folders containing the datasets for the temperature ranges indicated. The individual file names can be used to deduce the temperature for that particular measurement. For example, the sub-folder 50-145_degC contains 20 datasets for which the first (run 895262) relates to 50$^o$C and the last (run 895281) to 145$^o$C. All data files have had background due to the quartz tube subtracted. The inclusion of run number in the file name means that they can immediately be used for sequential refinement using seqgsas. The Mössbauer data contain two columns that correspond with the experimental x-axis (mm s-1) and y-axis (−absorption / %) points for the collected raw data. The heating temperature is included in the file name

Topotactic fluorine insertion into the channels of FeSb2O4-related materials.

This paper discusses the fluorination characteristics of phases related to FeSb2O4, by reporting the results of a detailed study of Mg0.50Fe0.50Sb2O4 and Co0.50Fe0.50Sb2O4. Reaction with fluorine gas at low temperatures (typically 230 C) results in topotactic insertion of fluorine into the channels, which are an inherent feature of the structure. Neutron powder diffraction and solid state NMR studies show that the interstitial fluoride ions are bonded to antimony within the channel walls to form Sb – F – Sb bridges. To date, these reactions have been observed only when Fe2+ ions are present within the chains of edge-linked octahedra (FeO6 in FeSb2O4) that form the structural channels. Oxidation of Fe2+ to Fe3+ is primarily responsible for balancing the increased negative charge associated with the presence of the fluoride ions within the channels. For the two phases studied, the creation of Fe3+ ions within the chains of octahedra modify the magnetic exchange interactions to change the ground-state magnetic symmetry to C-type magnetic order in contrast to the A-type order observed for the unfluorinated oxide parents

Probing the Mechanism of Sodium Ion Insertion into Copper Antimony Cu₂Sb Anodes

We report experimental studies to understand the reaction mechanism of the intermetallic anode Cu₂Sb with Na and demonstrate that it is capable of retaining about 250 mAh g^–1 over 200 cycles when using fluoroethylene carbonate additive. X-ray diffraction data indicate during the first discharge the reaction leads to the formation of crystalline Na₃Sb via an intermediate amorphous phase. Upon desodiation the Na₃Sb reverts to an amorphous phase, which then recrystallizes into Cu₂Sb at full charge, indicating a high degree of structural reversibility. The structure after charging to 1 V is different from that of Cu₂Sb, as indicated by X-ray absorption spectroscopy and ¹²¹Sb Mössbauer spectroscopy, and is due to the formation of an amorphous Na–Cu–Sb phase. At full discharge, an isomer shift of −8.10 mm s^–1 is measured, which is close to that of a Na₃Sb reference powder (−7.95 mm s^–1) and in agreement with the formation of Na₃Sb domains. During charge, the isomer shift at 1 V (−9.29 mm s^–1) is closer to that of the pristine material (−9.67 mm s^–1), but the lower value is consistent with the lack of full desodiation, as expected from the potential profile and the XAS data