A mechanism was established for the formation of nanosized iron carbide particles encapsulated in carbon shells via the processes of ferrocene thermal conversions at high pressures. At a pressure of ...8.0 GPa, products of ferrocene decomposition were studied as a function of temperature by X-ray diffraction, Raman and Mössbauer spectroscopy, scanning and transmission electron microscopy. It was shown that the mechanism of formation of the carbon-encapsulated iron carbide nanoparticles at high pressures and temperatures differs qualitatively from the known mechanism of their formation in the gas-phase processes of laser pyrolysis or photolysis of ferrocene. At high pressures and temperatures, the formation of iron carbide nanoparticles occurs not due to the primary growth of pure iron particles and the subsequent dissolution of carbon in iron. Nanoparticles are formed due to the direct fusion of iron–carbon clusters, which are formed at intermediate stages of ferrocene thermal destruction. Then, obtained amorphous iron carbides Fe1–x C x with a high carbon content start to crystallize. Two crystalline carbon-encapsulated forms of iron carbide (Fe7C3 and Fe3C) are the main products of crystallization of the amorphous Fe1–x C x depending on the temperature of the ferrocene treatment.
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•Precision studies of the temperature dependence of the Mössbauer spectra of FeBO3 single crystals have been carried out.•Parameters of hyperfine interaction in FeBO3 a wide temperature range were ...accurately determined.•Theoretical analysis of the peculiarities of the hyperfine structure formation of Mössbauer spectra in iron borate was performed.•A technique for correcting Mössbauer spectra taking into account the effective thickness of the absorber was developed.•Debye temperature for cations in the structure of FeBO3 was determined.
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In this work, Mössbauer spectroscopy and X-ray diffraction were used to determine the precision values of the hyperfine interaction parameters and the crystal structure of FeBO3 single crystals in a wide temperature range, including the region of magnetic phase transitions (TN). A theoretical model has been developed to describe nuclear resonance transitions in iron atoms in the approximation of a combined magnetic dipole and electric quadrupole hyperfine interaction. A technique for correcting the Mössbauer spectra, considering the effective thickness of the absorber, has also been elaborated.
It is shown that the appearance of two additional resonance lines in the hyperfine structure significantly affects the shape of the FeBO3 spectra near the N é el temperature (ТN). The characteristic Debye temperatures for Fe and B cations in the iron borate structure have been determined. The developed technique and the results obtained are extremely important for the use of FeBO3 crystals in new high-tech branches of science and technology, including optoelectronics and synchrotron technologies.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The process of interaction of graphene with iron oxide nanoparticles was investigated. First, graphene oxide (GO) modified with magnetite Fe3O4 nanoparticles was successfully synthesized. Raman and ...Mössbauer spectroscopy revealed that the magnetite Fe3O4 in combination with GO became non-stoichiometric, and the maghemite phase γ-Fe2O3 appears. Subsequent reduction of graphene oxide by thermal treatment leads to an increase in the fraction of maghemite content and, in addition, the hematite phase α-Fe2O3 appears in the sample annealed at above 500 °C. Meanwhile, the core-shell nanocomposites of FexOy/G appear, were FexOy consists of a mixture of the Fe3O4, γ-Fe2O3 and α-Fe2O3 phases. The content of each phase can be varied by the annealing temperature. Magnetic, Mössbauer and Raman spectroscopy measurements indicate that graphene can interact with iron oxide. Charge-transfer from iron to graphene can occur due to delocalization of 3d electrons, which reduces the overall magnetic moment of the charge-transfer complexes. These properties can have potential applications in electronic such as supercapacitors, advanced anode materials for lithium-ion batteries, magnetically targeted drug delivery, photothermic therapy, and magnetic resonance imaging.
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•Graphene modified by iron oxide nanoparticles FexOy/G was successfully synthesized.•The nanocomposites FexOy/G have a core-shell type design.•High temperature treatment leads to reduction of the GO and γ-Fe2O3 formation.•Magnetite act as electron donors to graphene, and charge-transfer interaction between graphene and iron oxide takes place.•Charge-transfer occurs due to delocalization of 3d electrons in the interface layer of the core-shell nanocomposites.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The core@shell nanostructures were obtained in the process of transformation of ferrocene Fe(C5H5)2 at high pressure (HP) of 8 GPa and high temperature (HT) of 900 °C with an isothermal exposure time ...t varying from 10 to 10000 s. At t > 300 s, the iron carbide o-Fe7C3 nanoparticles with an orthorhombic crystal structure (sp.gr. Pnma) can be created, which are dispersed in highly defective carbon matrix. After opening the high-pressure cell, a series of redox reactions occurs, leading to a formation of iron oxides on the surface of the iron carbide core. When the size of Fe7C3 nanoparticle is less than critical one the nanoparticle is fully oxidized, while in the larger particle an amorphous iron oxide shell is formed. A sequential increase in t initiates crystallization processes both in the iron carbide subsystem and in the carbon subsystem, resulting in the formation of core@shell Fe7C3/FexOy/C structures. Iron oxides with a cubic spinel-type structure (Fe3O4/γ-Fe2O3) appear in the shell. However, under oxygen reduction, part of magnetite can be transformed into wüstite FeO. The magnetic properties of magnetite and wüstite are radically different, and by varying the thickness of these layers, structures with the desired functional properties can be obtained.
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Core-shell FexOy@C nanoparticles (NPs) modified with Ag were studied with x-ray diffraction, transmission electron microscopy, energy dispersive elemental mapping, Mössbauer spectroscopy, static ...magnetic measurements, and optical magnetic circular dichroism (MCD). FexOy@C NPs synthesized by the pyrolysis process of the mixture of Fe(NO3)3 · 9H2O with oleylamine and oleic acid were added to a heated mixture of oleylamine and AgNO3 in different concentrations. The final product was a mixture of iron oxide crystalline NPs in an amorphous carbon shell and Ag crystalline NPs. The iron oxide NPs were presented by two magnetic phases with extremely close crystal structures: Fe3O4 and γ-Fe2O3. Ag is shown to form crystalline NPs located very close to the iron oxide NPs. An assumption is made about the formation of hybrid FexOy@C-Ag NPs. Correlations were obtained between the Ag concentration in the fabricated samples, their magnetic properties and the MCD spectrum shape. Introducing Ag led to a approximately linear decrease of the NPs saturation magnetization depending upon the Ag concentration, it also resulted into the MCD spectrum shift to the lower light wave energies. MCD was also studied for the Fe3O4@C NPs synthesized earlier with the same one-step process using different heat treatment temperatures, and MCD spectra were compared for two series of NPs. A possible contribution of the surface plasmon excitation in Ag NPs to the MCD spectrum of the FexOy@C-Ag NPs is discussed.
•Parameters of the spin helicoid in multiferroic Ba3SbFe3Si2O14are obtained for all T < TN.•A physical model for the behavior of the Mössbauer spectra in spiral magnetic structures is developed.•This ...model gives fine information about complicated magnetic structures even in polycrystalline samples.•The magnetic anisotropy axis can be found in a triangle magnetic lattice.
A physico-mathematical model for processing the Mössbauer spectra of the langasite family compounds with a helical magnetic structure was developed. It was shown that the Mössbauer spectra demonstrate high information content and sensitivity to details of the magnetic structure even in the case of polycrystalline samples and in the absence of an external magnetic field. In addition, they make it possible to monitor the dynamics of the magnetic structure in the entire temperature range below the Néel point. As an example, the helical magnetic structure in the langasite Ba3SbFe3Si2O14 was experimentally investigated. It was established that during translation along the c axis, the magnetic moments of iron in the (ab) planes rotate at an angle of 72°, forming a spiral with a period of 5c. The directions of the main axis of the electric field gradient (EFG) and the local axis of magnetic anisotropy were determined, which are very important characteristics of the frustrated magnetic structure of langasites.
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•Core-shell Fe3O4@Au nanostructures obtained with an advanced two step synthesis.•Mössbauer data revealed interaction between iron and gold atoms at the interface.•Gold covering of Fe3O4 stabilizes ...the magnetic properties of magnetite.
Core-shell Fe3O4/Au nanostructures were obtained with an advanced method of two step synthesis and several complementary methodics were applied for investigation structural and magnetic properties of the samples. Along with X-ray diffraction and transmission electron microscopy, electron diffraction, optical, Raman and Mössbauer spectroscopy were used for nanoparticle characterization. It was established that the physical and structural properties Fe3O4/Au nanocomposites are specific of intrinsic properties of gold and magnetite. Mössbauer and Raman spectroscopy data indicated that magnetite was in a nonstoichiometric state with an excess of trivalent iron both in the initial Fe3O4 nanoparticles and in the Fe3O4/Au nanocomposites. As follows from the Mössbauer data, magnetic properties of iron ions in the internal area (in core) and in the surface layer of magnetite nanoparticles are different due to the rupture of exchange bonds at the particles surface. This leads to decrease in an effective magnetic moment at the surface. Gold atoms at the interface of the composites interact with dangling bonds of magnetite and stabilize the magnetic properties of the surface layers of magnetite.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Iron sulfide nanoparticles Fe
3
S
4
with the spinel-type crystal structure were synthesized by the polyol mediated process. The particle size depends on preparation conditions and varies from 9 to ...20 nm. Mössbauer data have revealed that the dominating fraction of iron ions in the 9-nm sample is in the high-spin ferric state. This implies an occurrence of the cation vacancies in nonstoichiometric greigite. The stoichiometric phase of greigite Fe
3
S
4
dominates in the 18-nm-size nanoparticles. Magnetic measurements have shown a ferrimagnetic behavior of all samples at temperatures between 78 and 300 K. The estimated value of magnetic moment of the stoichiometric greigite nanoparticles is about 3.5 μ
B
per Fe
3
S
4
unit. The Mössbauer spectra indicate a superparamagnetic behavior of small particles, and some fraction of superparamagnetic phase is observed in all samples synthesized which may be caused by the particle size distribution. The blocking temperatures of
T
B
≈ 230 and 250 K are estimated for the 9 and 14 nm particles, respectively. The Mössbauer parameters indicate a great degree of covalency in the Fe–S bonds and support the fast electron Fe
3+
⇆ Fe
2+
exchange in the B-sites of greigite. An absence of the Verwey transition at temperatures between 90 and 295 K is established supporting a semimetal type of conductivity. The temperature and magnetic field dependences of the magnetic circular dichroism (MCD) of optical spectra were measured in Fe
3
S
4
for the first time. The spectra differ substantially from that of the isostructural oxide Fe
3
O
4
. It is supposed that the MCD spectra of greigite nanoparticles result from the collective electron excitations in a wide band with superimposed peaks of the
d
–
d
transitions in Fe ions.
•Single-crystalline nanoparticles of nickel-chromium ferrite with size of 5–50 nm were synthesized.•The cation distribution (Fe0.75 Ni0.25) Ni0.75 Cr1.25 O4 was established.•The canted magnetic ...structure in the octahedral B- sublattice results in magnetic compensation.•Frustrated magnetic ordering is caused by AFM and FM competition exchange interactions.•The atomic-scale effect of exchange coupling of “soft” and “hard” magnetic B- and A- sublattices was discovered.
A set of single-crystalline nanoparticles (NPs) of nickel-chromium ferrite NiFe0.75Cr1.25O4 with a cubic spinel structure were synthesized and investigated. The NPs size can be varied from about 5 to 50 nm by the final annealing of the precursor at different temperatures. The distribution of cations over the tetrahedral (A) and the octahedral B sites (Fe0.75 Ni0.25) Ni0.75 Cr1.25 O4 was established from the magnetic and Mössbauer measurements. In large NPs, the magnetic structure at low temperatures is close to the collinear antiferromagnetic (AFM) structure of the Neel type; and the total magnetic moment Mtot of the ferrite coincides with the direction of the B-sublattice moment. Several size-dependent magnetic anomalies were revealed. Three types of magnetic ions present in the A- and B- sublattices cause the competition of AFM and FM exchange interactions resulting in the highly frustrated magnetic ordering and the occurrence of canted magnetic structure in the octahedral B-sublattice. The frustrated structure is very flexible and significantly subjected to temperature and applied field. It results in several magnetic anomalies observed, including the occurrence of magnetic compensation, abnormal behavior of ZFC and FC magnetization curves and hysteresis loops. It was shown that magnetic anomalies can be explained in terms of exchange coupling of “soft” and “hard” magnetic B- and A-sublattices. This effect in the (Fe0.75 Ni0.25) Ni0.75Cr1.25 O4 NPs can be considered as an atomic-scale analog of a similar effect observed in two-phase exchange-coupled alloys developed for permanent magnets and for the perpendicular recoding media.
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A van der Waals telluride, NbFeTe2, has been synthesized using chemical vapor transport reactions. The optimized synthetic conditions yield high-quality single crystals with a novel monoclinic ...crystal structure. Monoclinic NbFeTe2 demonstrates a (100) cleavage plane, bulk ferromagnetism below 87 K, and a metallic ground state—the necessary prerequisites for needed spintronics technologies.
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