Nanocrystallinity induced by heating

Samples of akaganeite (β-FeOOH) and goethite (α-FeOOH) have been studied after heating at various temperatures up to 800 °C. X-ray diffraction and Mössbauer spectroscopy measurements showed that slightly below the temperatures at which the samples transform to hematite (α-Fe₂O₃) the oxyhydroxide phases become nanocrystalline.     

Single-Phase Precursors for the Preparation of Spinel Ferrites via Oxalate Route: the Study of Cobalt Ferrite Synthesis

This work explores the benefits of single-phase oxalate precursors for the preparation of spinel ferrites by thermal decomposition. A direct comparison between the genuine oxalate solid solution and the physical mixture of simple oxalates is presented using the case study of cobalt ferrite preparation. The mixing of metal cations within a single oxalate structure could be verified prior to its thermal decomposition by several non-destructive experimental techniques, namely Mössbauer spectroscopy, X-ray powder diffraction (XRD) and energy-dispersive X-ray spectroscopy. In situ XRD experiments were conducted to compare the decomposition processes of the solid solution and the physical mixture. Additionally, the decomposition products of the FeCo oxalate solid solution were studied ex situ by means of N₂ adsorption, Mössbauer spectroscopy and XRD. The results obtained for different reaction temperatures demonstrate the possibilities to easily control the physical properties of the prepared oxides.     

Thermal Analysis of Magnesium Tris(maleato) Ferrate(III) Dodecahydrate. Physico-chemical studies

Thermal analysis of magnesium tris(maleato) ferrate(III) dodecahydrate has been studied from ambient to 700°C in static air atmosphere employing TG, DTG, DTA, XRD, Mössbauer and infrared spectroscopic techniques. The precursor decomposes to iron(II) intermediate species along with magnesium maleate at 248°C. The iron(II) species then undergo oxidative decomposition to give α-Fe₂O₃ at 400°C. At higher temperatures magnesium maleate decomposes directly to magnesium oxide, MgO, which undergoes a solid state reaction with α-Fe₂O₃ to yield magnesium ferrite (MgFe₂O₄) at 600°C, a temperature much lower than for ceramic method. The results have been compared with those of the oxalate precursor.     

A simple solution route to control synthesis of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanomaterials at low temperature and their magnetic properties

A series of nanostructured iron compounds including cubic Fe₃O₄ and orthorhombic FeOOH were synthesized via a facile low temperature (in the range of 60?100°C) solution method. In the whole process, the interaction between FeCl₂·4H₂O and methenamine (C₆H₁₂N₄) was carried out through a reflux device under different reaction conditions such as temperature, solvent, and duration. The samples were detected by XRD, TEM, SAED, physical property measurement system, and Mössbauer spectroscopy, separately. The experiments showed that magnetic mixture nanoparticles had flake and rod morphologies, and cubic Fe₃O₄ took on grain nanostructure. Magnetism measurements indicated that the saturated magnetization of the as-obtained magnetic mixture was lower than that of the cubic magnetite. Mössbauer spectroscopy testified the sample consisting of cubic magnetite rather than γ-Fe₂O₃. In addition, a possible growth mechanism of cubic magnetic nanoparticles under different conditions was discussed.     

Synthesis of lithium ferrite by precursor and combustion methods: A comparative study

The thermal decomposition of lithium hexa(carboxylato)ferrate(III) precursors, (Li₃[Fe(L)₆]·xH₂O, L = formate, acetate, propionate, butyrate), has been carried out in flowing air atmosphere from ambient temperature upto 500 °C. Various physico-chemical techniques, i.e., TG, DTG, DTA, XRD, SEM, IR, Mössbauer spectroscopy, etc., have been employed to characterize the intermediates and end products. After dehydration, the anhydrous complexes undergo decomposition to yield various intermediates, i.e., lithium oxalate/acetate/propionate/butyrate, ferrous oxalate/acetate and α-Fe₂O₃ in the temperature range of 185–240 °C. A subsequent decomposition of these intermediates leads to the formation of nanosized lithium ferrite (LiFeO₂). Ferrites have been obtained at much lower temperature (255–310 °C) as compared to conventional ceramic method. The same nano-ferrite has also been prepared by the combustion method at a comparatively lower temperature (400 °C) and in less time than that of conventional ceramic method.     

Mössbauer 237Np and crystallographic studies of MIINpF6·3H2O (MII = Mn,Fe, Co) compounds

The compounds M^IINpF₆3H₂O with M^II = Mn, Fe, Co were prepared as single crystals by hydrothermal synthesis (T = 400°C, P = 2000 bars). CoNpF₆3H₂O crystallises in a monoclinic system with C2 space group. Cell parameters are a = 12.143(9) ā; b = 6.922(5) ā; c = 7.942(5) ā; β = 92.84°.The Mössbauer measurements were performed in a conventional He Cryostat. The Mössbauer source used in the experiments was a 500 mCi ²⁴¹Am metal with a conventional sine mode drive system.A microbalance magnetometer attached to a varying temperature Cryostat was used for the susceptibility measurements. The maximum applied magnetic field was 14KG.The Mössbauer spectroscopy of ²³⁷Np shows a magnetically split hyperfine spectrum at 4.2K for all those compounds.The spectra can be fitted with a magnetic hyperfine field associated to a quadrupole splitting using the linear correlation between B_eff and e²qQ. From isomer shift measurements, we confirm the IV charge state of Np in these 3 compounds.The magnetic susceptibility shows antiferromagnetic type transition. 1/ξ = f(T) follows a Curie-Weiss law above T_N.     

A facile carbothermal preparation of Sn–Co–C composite electrodes for Li-ion batteries using low-cost carbons

Sn–Co–C composites were prepared by carbothermal reaction of ball-milled precursors. X-ray diffraction and ¹¹⁹Sn Mössbauer spectroscopy of the original composites revealed the predominance of Sn and CoSn₂ phases for those samples prepared with a high Sn/Co ratio. Electron microscopy images showed a homogeneous dispersion of sub-micrometric metallic particles in the carbon matrix. Galvanostatic cycling at several kinetic rates revealed that Sn₇Co₁C₉₂ and Sn₈Co₄C₈₈ are able to maintain 400 mA h g^?1 after 40 cycles at 35 mA g^?1. The large CoSn₂ contribution revealed by Mössbauer spectroscopy in the original and cycled electrodes is responsible for the good electrochemical performance. This interpretation is also supported by impedance spectroscopy measurements.     

Preparation of MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles by microemulsion method and their characterization

This work presents the preparation and characterization of magnesium ferrite which is one of the important magnetic oxides with spinel structure. Magnesium ferrite was prepared via microemulsion method mediated hydrolytic decomposition of mixed alkoxide solutions. This microemulsion was using for preparation magnesium ferrit for the first time. The starting solution, composed from magnesium methoxide and iron ethoxide in dry ethanol, was introduced in to the prepared microemulsion and sequentially hydrolyzed by distilled water addition (Pithan et al. in J Cryst Growth 280:191–200, 2005; Shiratori et al. in J Eur Ceram Soc 25:2075–2079, 2005; Herrig and Hempelmann in Mater Lett 27:287–292, 1996). After raw powder precipitation, the samples were decantanted by ethanol and then calcined at temperatures 800, 900, 1,000 or 1,100 °C for 1 h. The resulting samples were characterized using powder X-ray diffraction, high resolution transmission electron microscopy, Mössbauer spectroscopy and magnetic measurements. X-ray diffraction and Mössbauer spectroscopy confirmed the presence of the spinel phase. The particles size was calculated from the XRD line broadening using Scherrer equation and their size was found about 31–38 nm, with only slight dependence on the heat treatment temperature. TEM revealed the particles size of about 39 nm. Magnetic measurements showed a ferrimagnetic behavior for all samples.     

Physico-chemical Studies on Thermal Decomposition of Alkali Tris(maleato)ferrates(III)

The thermal decomposition of alkali tris(maleato)ferrates(III), M₃ [Fe(C₂ H₂ C₂ O₄ )₃ ] (M =Li, Na, K) has been studied isothermally and non-isothermally employing simultaneous TG-DTG-DTA, XRD, Mössbauer and IR spectroscopic techniques. The anhydrous complexes decompose in the temperature range 215–300°C to yield Fe(II)maleate as an intermediate followed by demixing of the cations forming α-Fe₂ O₃ and alkali metal maleate/oxalate in successive stages. In the final stage of remixing of the cations (430–550°C) a solid state reaction occurs between α-Fe₂ O₃ and alkali metal carbonate leading to the formation of fine particles of respective ferrites. The thermal stabilities of the complexes have been compared with that of alkali tris(oxalato)ferrates(III).     

Insertion of iron oxide in calciumsilicate gel-derived materials

Samples of the composition of 10Fe₂O₃·10CaO·80SiO₂ were prepared by the sol-gel method and heat-treated in different atmospheres. They were investigated by X-ray diffraction, scanning electron microscopy and Mössbauer spectroscopy. In the heat-treated samples in air iron is present up to 1000 °C in form of hematite and as Fe³⁺ in the tetrahedral sites. A wide range of hematite particle sizes was observed, the average size increased with heating temperature. At 1000 °C wollastonite was observed, at 1200 °C tridymite was formed and all the iron was incorporated in hematite. A heat-treatment at 500 °C under reducing conditions led to poorly crystallized maghemite and at 700 °C to metallic iron and fayalite formation.     

A comparative study on the thermal decomposition of some transition metal carboxylates

Thermal decomposition of transition metal malonates, MCH₂C₂O₄?xH₂O and transition metal succinates, M(CH₂)₂C₂O₄?xH₂O (M=Mn, Fe, Co, Ni, Cu, Zn) has been studied employing TG, DTG, DTA, XRD, SEM, IR and Mössbauer spectroscopic techniques. After dehydration, the anhydrous metal malonates and succinates decompose directly to their respective metal oxides in the temperature ranges 310–400 and 400–525°C, respectively. The oxides obtained have been found to be nanosized. The thermal stability of succinates have been found to be higher than that of the respective malonates.     

Novel sol–gel method for preparation of high concentration ε-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> nanocomposite

ε-Fe₂O₃/SiO₂ nanocomposite was prepared by novel solgel method using single precursor for both nanoparticles and matrix. This method allows to prepare the samples free of α-Fe₂O₃ with 40% of Fe₂O₃ in SiO₂. Nanoparticles of 12 nm diameter were obtained by annealing at 1,000 °C. The samples were characterized by powder X-ray diffraction and transmission electron microscopy. Mössbauer spectroscopy identified ε-Fe₂O₃ as the only magnetically ordered phase at room temperature. Magnetic measurements revealed progressive necking of hysteresis loops measured at 300 and 2 K. In both cases the intrinsic coercivity reaches only 0.25 T. Measurements up to 14 T shows monotonous decreasing trend of saturated magnetization with increasing temperature.     

Synthesis of potassium ferrite by precursor and combustion methods

The thermolysis of potassium hexa(carboxylato)ferrate(III) precursors, K₃[Fe(L)₆]·xH₂O (L=formate, acetate, propionate, butyrate), has been carried out in flowing air atmosphere from ambient temperature to 900°C. Various physico-chemical techniques i.e. TG, DTG, DTA, XRD, IR, Mössbauer spectroscopy etc. have been employed to characterize the intermediates and end products. After dehydration, the anhydrous complexes undergo exothermic decomposition to yield various intermediates i.e. potassium carbonate/acetate/propionate/butyrate and α-Fe₂O₃. A subsequent decomposition of these intermediates leads to the formation of potassium ferrite (KFeO₂) above 700°C. The same ferrite has also been prepared by the combustion method at a comparatively lower temperature (600°C) and in less time than that of conventional ceramic method.     

Crystal structure and the magnetic properties of tris(2-chloromethyl-4-oxo-4H-pyran-5-olato-κ2O5,O4)iron(III)

The tris(2-chloromethyl-4-oxo-4H-pyran-5-olato-κ²O⁵,O⁴)iron(III), [Fe(kaCl)₃], has been synthesized and characterized by the crystal structure analysis, magnetic susceptibility measurements, Mössbauer, and EPR spectroscopic methods. The X-ray single crystal analysis of [Fe(kaCl)₃] revealed a mer isomer. The magnetic susceptibility measurements indicated the paramagnetic character in the temperature range of 2 K–298 K. The EPR and Mössbauer spectroscopy confirmed the presence of an iron center in a high-spin state. Additionally, the temperature-independent Mössbauer magnetic hyperfine interactions were observed down to 77 K. These interactions may result from spin–spin relaxation due to the interionic Fe³⁺ distances of 7.386 Å.     

Crystal structure and magnetic properties of tris(2-hydroxymethyl-4-oxo-4H-pyran- 5-olato-κ2O5,O4)iron(III)

Tris(2-hydroxymethyl-4-oxo-4H-pyran-5-olato-κ²O⁵,O⁴)iron(III) [Fe(ka)₃], has been characterised by magnetic susceptibility measurements Mössbauer and EPR spectroscopy. The crystal structure of [Fe(ka)₃] has been determined by powder X-ray diffraction analysis. Magnetic susceptibility and EPR measurements indicated a paramagnetic high-spin iron centre. Mössbauer spectra revealed the presence of magnetic hyperfine interactions that are temperature-independent down to 4.2?K. The interionic Fe³⁺ distance of 7.31?Å suggests spin-spin relaxation as the origin of these interactions.     

Preparation of magnetic polystyrene latex via the miniemulsion polymerization technique

Magnetic iron oxide (magnetite, Fe₃O₄) nanoparticles were encapsulated with polystyrene to give a stable water‐based magnetic polymer latex, using the miniemulsion polymerization technique. The resulting magnetic latexes were characterized with transmission electron microscopy (TEM), dynamic light scattering (DLS), vibrating sample magnetometer measurements (VSM), and ⁵⁷Fe Mössbauer spectroscopy measurements. TEM revealed that all magnetite nanoparticles were embedded in the polymer spheres, leaving no empty polystyrene particles. The distribution of magnetite particles within the polystyrene spheres was inhomogeneous, showing an uneven polar appearance. The DLS measurements indicated a bimodal size distribution for the particles in the latexes. According to our magnetometry and Mössbauer spectroscopy data, the encapsulated magnetite particles conserve their superparamagnetic feature when they are separated in the polymer matrix. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4802–4808, 2004     

Mössbauer study of the thermal decomposition of alkali tris(oxalato)ferrates(III)

The thermal decomposition of alkali (Li,Na,K,Cs,NH₄) tris(oxalato)ferrates(III) has been studied at different temperatures up to 700°C using Mössbauer, infrared spectroscopy, and thermogravimetric techniques. The formation of different intermediates has been observed during thermal decomposition. The decomposition in these complexes starts at different temperatures, i.e., at 200°C in the case of lithium, cesium, and ammonium ferrate(III), 250°C in the case of sodium, and 270°C in the case of potassium tris(oxalato)ferrate(III). The intermediates, i.e., Fe¹¹C₂O₄, K₆Fe¹¹₂(ox)₅. and Cs₂Fe¹¹ (ox)₂(H₂O)₂, are formed during thermal decomposition of lithium, potassium, and cesium tris(oxalato)ferrates(III), respectively. In the case of sodium and ammonium tris(oxalato)ferrates(III), the decomposition occurs without reduction to the iron(II) state and leads directly to α-Fe₂O₃.     

Local structure analysis of Bi2(Ga1-xFex)4O9 mullite-type solid solutions synthesized via combination of ball milling and thermal treatment

Mullite-type Bi₂(Ga_1-xFe_x)₄O₉ solid solutions, with 0.1 ≤ x ≤ 0.9, have been synthesized by a combination of mechanical and thermal treatments of a Bi₂O₃/Ga₂O₃/α-Fe₂O₃ stoichiometric mixture. The microstructure of the as-prepared materials on the long-range and local atomic scales was investigated by X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy, respectively. The XRD data analysis revealed in all cases linear dependence of the lattice parameters related on x. Due to the ability of the applied Mössbauer spectroscopy to probe the local environment of Fe cations, the local structural disorder in investigated solid solutions is provided. It was shown that the presence of Fe³⁺ cations in octahedral sites of the orthorhombic structure causes a local distortion of polyhedra in the material. The preferential occupation of Fe in octahedral site was revealed. Detailed quantitative information on both the cation distribution and the bond lengths provided is discussed in relation to the derived hyperfine parameters.     

Synthesis and characterization of nanocrystalline zinc aluminate spinel by sol–gel technique using modified alkoxide precursor

Nanosized zinc aluminate spinel (gahnite, ZnAl₂O₄) powders were prepared by sol−gel technique at low sintering temperatures. Aluminium-sec-butoxide [Al(O^sBu)₃] and zinc nitrate hexahydrate Zn(NO₃)₂ . 6H₂O were used as starting materials. Gels with and without chelating agent were prepared. Ethyl-acetoacetate (C₆H₁₀O₃) was used as a chelating agent in order to control the rate of hydrolysis of Al(O^sBu)₃. The dried gels and thermally treated samples were characterized by means of Differential Thermal Analysis and Thermo-Gravimetric Analysis (DTA, TGA), X-ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR) and Transmission Electron Microscopy (TEM). The surface area was measured by Brunauer-Emmet-Teller (BET) adsorption–desorption isotherms. It has been established that chelation enables to obtain a transparent gel. The thermal evolution of gels was characterized by two crystallization processes in the range 200–400 °C and 600–700 °C. Both processes yielded pure ZnAl₂O₄ as evidenced by XRD, i.e. zinc aluminate spinel powders were produced by gel heat-treatment at temperatures as low as 300 °C. The average gahnite crystallite size for the samples sintered in the temperature range of 400–1000 °C has been calculated from the broadening of XRD lines revealing that nanocrystalline powders were prepared. The surface areas measured for the samples fired at 700 °C for 2 h were 43.1 and 62.6 m² g⁻¹, for sample without and with the chelating agent, respectively. TEM micrographs confirmed the nano-scale size of particles.     

Template synthesis of ε-Fe2O3 nanoparticles in opal-like matrices

Nanoparticles of ε-Fe₂O₃ were obtained by template synthesis in the pores of opal-like matrices. The maximum content of ε-Fe₂O₃ is achieved upon calcination at 1000 °C for 2–4 h. The content of ε-Fe₂O₃ in a mixture of modifications of iron oxides reaches 80–90%, which is confirmed by the data of X-ray diffraction analysis, Mössbauer spectroscopy and magnetometry experiments.