Multiscale structural changes of atomic order in severely deformed industrial aluminum

The regularities of multiscale structural changes in the atomic order of the aluminum alloy AD-1 after a severe cold plastic deformation by conventional rolling in smooth rolls or in rolls with relief recesses favorable for shear deformation have been investigated. It has been found that there are four types of structural fractions that differ in scale and perfection of atomic order: crystallographic planes with a long-range order; nanoscale fragments of the planes (D = 100–300 Å) with an incipient long-range order; smaller groups of atoms (D = 20–30 Å) of amorphized structure; and the least ordered structural fraction of intercluster medium, keeping only a short-range atomic order (2–3 interatomic distances, 10 Å). The presence of diffuse halo bands in the region of intense Debye lines indicates phase transitions of the order → disorder type with the formation of one to three groups of amorphous clusters with the dominance, in the nanometer scale, of the atomic order characteristic of the family of planes (111), (220), and (311) of crystalline aluminum. We have found a dynamic phase transition with the changing crystallographic order of aluminum, with the matrix structure of a face-centered cubic (FCC) lattice, in the form of nanosized local groups of atoms, that is, the deformation clusters of aluminum with a simple cubic K6 lattice. In the case of conventional rolling, the development of large clusters 50–500 Å in size is observed; however, in the use of rolls with relief recesses, the difference in the sizes of the clusters is one half as much: 50–250 Å. Based on the analysis of the integrated intensity of incoherent X-ray scattering by the samples, we have elucidated the nature of the lowest measured density for the sample subjected to conventional rolling, which consists in the volume concentration of disorderly arranged atoms, the highest of the compared structures, which indicates the formation therein of the greatest amount of fluctuation “voids.”     

Phase composition and the structure of (1–<Emphasis Type="Italic">x</Emphasis>)KNO<Subscript>3</Subscript> + <Emphasis Type="Italic">x</Emphasis>Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposites by X-ray diffraction

The phase composition and the structural properties of potassium nitrate KNO₃ and its heterogeneous composites with nanometer-sized powder of aluminum oxide Al₂O₃ have been studied by X-ray diffraction at various concentrations of an Al₂O₃ nanopowder. It is found that, in the (1–x)KNO₃ + xAl₂O₃ nanocomposites, additional high-temperature rhombohedral phase of potassium nitrate (phase III) with lattice parameters a = 5.4644 Å and c = 9.0842 Å. With increasing concentration of Al₂O₃ nanopowder, the content of the main potassium nitrate phase (phase II) is found to significantly decrease, and the relative fraction of the phase III in the total content of the nitrate in the composite composition increases. This phase is assumed to be “frozen” in the nanocomposite at the KNO₃–Al₂O₃ interface. The estimated size of KNO₃ crystallites in the phase III is more than 20 nm.     

Magnetic-dichroism study of iron silicides formed at the Fe/Si(100) interface

The interplay between the phase composition, electronic structure, and magnetic properties of the Fe/Si(100)2×1 interface has been studied at the initial stages of its formation (at Fe doses up to 8 Å). The experiments were carried out in ultra high vacuum by using high-resolution photoelectron spectroscopy with synchrotron radiation. The interface magnetic properties were examined in terms of magnetic linear dichroism in angle-resolved Fe 3p core-level photoemission. It was found that at room temperature a disordered Fe–Si solid solution is formed at the first stage of Fe deposition (≤3.4 Å). In the coverage range of 3.4–4.3 Å the solid solution transforms into Fe₃Si. However, the in-plane ferromagnetic ordering of the silicide occurs only at 6.8 Å Fe that demonstrates the thickness dependence of the magnetic properties of Fe₃Si. The subsequent sample annealing to 150°C transforms Fe₃Si to ε-FeSi, leading to the disappearance of ferromagnetic behavior.     

The structure of atomic nitrogen adsorbed on Fe(100)

Nitrogen atoms adsorbed on a Fe(100) surface cause the formation of an ordered c(2 × 2) overlayer with coverage 0.5. A structure analysis was performed by comparing experimental LEED I–V spectra with the results of multiple scattering model calculations. The N atoms were found to occupy fourfold hollow sites, with their plane 0.27 Å above the plane of the surface Fe atoms. In addition, nitrogen adsorption causes an expansion of the two topmost Fe layers by 10% (= 0.14 Å). The minimum r-factor for this structure analysis is about 0.2 for a total of 16 beams. The resulting atomic arrangement is similar to that in the (002) plane of bulk Fe₄N, thus supporting the view of a “surface nitride” and providing a consistent picture of the structural and bonding properties of this surface phase.     

Impacts of neodymium on structural,spectral and dielectric properties of LiNi0.5Fe2O4 nanocrystalline ferrites fabricated via micro-emulsion technique

Soft ferrites are technologically advanced smart materials and their properties can be tailored by controlling the chemical composition and judicial choice of the metal elements. In this article we discussed the effect of rare earth neodymium (Nd³⁺) on various properties of LiNi_0.5Nd_xFe_2−xO₄ spinel ferrites. These ferrites have been synthesized by facile micro-emulsion route and characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), a.c. electrical conductivity and thermal analysis. The influence of Nd³⁺ doping on structural and electrical parameters has been investigated. XRD analysis revealed the formation of single cubic spinel structure for x≤0.07. Few traces of secondary phase (NdFeO₃) were found for x≥0.105. The secondary phase induced owing to the solubility limit of Nd³⁺ cations in these ferrites. The lattice parameter (a) and crystallite size (D) both exhibit non-linear relation. The values of “a” and “D” were found in the range 8.322–8.329 Å and 25–32 nm respectively. These variations were attributed to the larger ionic radius of Nd³⁺ cations as compared to the host cations and lattice strain produced in these ferrites. The dielectric parameters were studied in the range 1 MHz to 3 GHz and these parameters were damped by Nd³⁺ incorporation and also by increasing the frequency. The reduced dielectric parameters observed in wide frequency range proposed that these nanocrystalline ferrites are potential candidates for fabricating the devices which are required to operate at GHz frequencies.     

Phase transitions in the CN<Subscript>x</Subscript>-TiN system attendant on the variation of the bias voltage during ion-stimulated growth

The structure of amorphous CN_x-TiN films grown by ion-stimulated deposition at a bias voltage U = 200–500 V is studied by X-ray diffraction. As the bias voltage increases in the range U = 300–360 V, the CN_x-TiN films are shown to undergo a phase transition in the amorphous phase having different order scales (20–50 Å): this transition is related to an increase in the content of the fraction of medium-cell (4 Å) carbon clusters as compared to the fractions of clusters with large (8 Å) and small (2 Å) cells. Under these conditions, 80–150 Å crystalline clusters undergo the phase transition from the Ti₂C(N) carbide into graphite (C _g) and diamond (C_d); the last two phases are represented by 100-Å clusters.     

Mössbauer studies of mixed-valence spin-crossover iron complexes with a hexadentate tripod ligand

The spin-crossover behaviors of mixed-valence iron compounds [Fe^IIH₃L][Fe^IIIL](NO₃)₂ (1) and [Fe^IIH₃L^Me][Fe^IIIL^Me](NO₃)₂ (2) have been investigated by ⁵⁷Fe Mössbauer spectroscopy, where H₃L is a hexadentate N₆ tripod ligand containing three imidazole groups and H₃L^Me is its 2-methylimidazole derivative. Deconvolution analyses of the Mössbauer spectra revealed that a two-step SCO (LS Fe^II–LS Fe^III→HS Fe^II–LS Fe^III→HS Fe^II–HS Fe^III) proceeds in each compound on elevating the temperature. Compound 2 exhibited lower spin-transition temperatures than 1. “Frozen-in effect” was observed below 120 and 50 K for 1 and 2, respectively.     

XANES investigations of interatomic interactions in multilayered nanostructures (Co45Fe45Zr10/a-Si)40 and (Co45Fe45Zr10/SiO2)32

The electronic structure and phase composition of amorphous multilayered nanostructures (Co₄₅Fe₄₅Zr₁₀/a-Si)₄₀ and (Co₄₅Fe₄₅Zr₁₀/SiO₂)₃₂ have been investigated by means of the X-ray absorption near-edge structure (XANES) technique, which is the most sensitive and useful in investigation of the chemical environment of elements in multicomponent nanostructures. The fact of interatomic interactions leading to the formation of composite “nanoferrite”-like FeO · Fe₂O₃ · ZrO₂(CoO) was established. Also it was shown that in the mentioned nanoferrite there is an exchange interaction which involves not only two- and three-charged ions of iron (Fe²⁺ and Fe³⁺) but also ions like Zr⁴⁺ and, partially, Co²⁺. The transformation of the thin structure of L _2,3-ranges for the iron component of multilayered nanostructures in XANES spectra reflects on the change of the ratio of di- and trivalent ions in iron oxides as a part of the composite “nanoferrite.”     

Magnetooptical,optical, and magnetotransport properties of Co/Cu superlattices with ultrathin cobalt layers

We investigated the field dependences of the magnetization and magnetoresistance of superlattices [Co(t _x, Å)/Cu(9.6 Å)]30 prepared by magnetron sputtering, differing in the thickness of cobalt layers (0.3 Å ≤ t _Co ≤ 15 Å). The optical and magnetooptical properties of these objects were studied by ellipsometry in the spectral region of hω= 0.09–6.2 eV and with the help of the transverse Kerr effect (hω= 0.5–6.2 eV). In the curves of an off-diagonal component of the tensor of the optical conductivity of superlattices with t _Co = 3–15 Å, a structure of oscillatory type (“loop”) was detected in the ultraviolet region, resulting from the exchange splitting of the 3d band in the energy spectrum of the face-centered cubic structure of cobalt (fcc Co). Based on magnetic experiments and measurements of the transverse Kerr effect, we found the presence of a superparamagnetic phase in Co/Cu superlattices with a thickness of the cobalt layers of 3 and 2 Å. The transition from superlattices with solid ferromagnetic layers to superparamagnetic cluster-layered nanostructures and further to the structures based on Co and Cu (t _Co = 0.3–1 Å) with a Kondo-like characteristics of the electrical resistivity at low temperatures is analyzed.     

Iodine adlayer structures on Au(111) as discerned by atomic-resolution scanning tunnelling microscopy: relation to iodide electrochemical adsorption

Coverage-dependent adlayer structures of iodine on Au(111) in air or organic solvents obtained by using a high-precision “beetle” — type scanning tunnelling microscope are reported in comparison with those obtained under potential control in aqueous electrochemical environments. The well-known (√3 × √3)R30° structure (Θ_I = 0.33) was only observed at the electrochemical interface at low potentials. Instead, two higher coverage adlattices are evident from the present STM images. The first comprises a (5 × √3) structure (Θ_I = 0.40), with a pair of iodine rows compressed by 20% in the R30° substrate direction. The second high-coverage phase (Θ_I ≈ 0.44) consists of a hexagonal iodine overlayer compressed and rotated a few degrees from the R30° direction, giving rise to a long-range (~ 20 Å) corrugation in the STM image. The virtues of quantitative atomic-resolution STM for deducing such complex adiayer structures are pointed out.     

Stability of the so-called “Co−Mo−S” phase in a carbonsupported Co−Mo sulfide catalyst at very low Co/Mo ratio

Several aspects of the stability of the so-called “Co?Mo?S” phase in a fully sulfided Co(0.04)Mo(6.84)/C (wt%) catalyst have been examined by Mössbauer emission spectroscopy. The “Co?Mo?S” parameters turned out to be unchanged when the catalyst is exposed for 23 days to ambient air at room temperature. However, if this exposure to air is extended for over 100 days, the so-called “Co?Mo?S” phase was found to be oxidized to a high-spin Fe²⁺ phase which has no strong interaction with the “MoS₂” particles. After resulfidation of the catalyst the so-called “Co?Mo?S” phase is formed again.     

57Fe Mössbauer spectra and magnetic data from the kagomé antiferromagnet H3O-jarosite

⁵⁷Fe Mössbauer spectra are presented from (H₃O)Fe₃(SO₄)₂(OH)₆ (or H₃O-jarosite), which is a model kagomé antiferromagnet which features geometrical frustration and spin-glass-like behaviour. Dynamic scaling of the freezing temperature as a function of frequency is observed over a large frequency range, which indicates the presence of a spin-glass transition. A fast relaxation model between “up” and “down” states, separated by an energy gap, is presented to account for the shape of the Mössbauer spectra below the freezing temperature. From a calculation of the Electric Field Gradient tensor, it is suggested that H₃O-jarosite is an XY-Heisenberg antiferromagnet, where the Fe³⁺ moments lie in the kagomé planes.     

T.E.M. study of Al2O3 metastable phases

Plasma spraying was employed to obtain rapidly solidified dense metastable alumina samples. They have been studied after being sprayed and in various annealed states by transmission electron microscopy and X-ray powder diffraction.

The so-called “γ” phase has been imaged by T.E.M. and exhibits a more or less ordered domain structure with quasi-periodic 1/4 <110> antiphase boundaries in the {100} planes of the defective spinel lattice.

Heating “γ” between 850 and 1050°C leads to more ordered intermediate phases. They are shown to appear through a two-dimensional antiphase periodic boundaries mechanism. Aluminum vacant sites are likely located along the antiphase planes and their concentration (Al_2.66□_0.33O₄) is consistent with the observed periodicities. The “δ” and “θ” forms are considered as variants of this structure.     

Swift heavy ion irradiation-induced modifications in structural and electrical properties of Y3+-substituted yttrium iron garnet

The consequences of 50 MeV Li³⁺ ion irradiation (fluence: 5×10¹³ ions/cm²) on the structural and electrical properties of the Y_3+xFe_5?xO₁₂ (x=0.0, 0.2, 0.4 and 0.6) garnet system have been investigated over the temperature range of 300–673 K. It is found that the percentage formation of an additional yttrium orthoferrite phase observed along with the bcc garnet phase considerably reduces for x=0.4 and 0.6 compositions after swift heavy ion (SHI) irradiation. The nature of thermal variation of DC resistivity curves for x=0.0 and 0.2 compositions is different from that for x=0.4 and 0.6 compositions. The SHI irradiation influences the magnitude of DC resistivity and conduction mechanism for the single-phase compositions while for mixed-phase compositions they remain unaffected. The results have been explained in the light of replacement of magnetic (5μ_B), smaller (0.64 Å), Fe³⁺ ion by nonmagnetic (0μ_B), larger (0.89 Å), Y³⁺ ion, the presence of the yttrium orthoferrite phase and swift heavy ion irradiation-induced paramagnetic centers in the system.     

Mössbauer study of proton radiation effects in FeCl24H2O

Abstract

The proton radiation effects in ferrous chloride are studied by means of the Mössbauer spectroscopy. The irradiation with protons of energy of 0.68 to 1.5 MeV has been found to cause dehydration and chemical decomposition of ferrous chloride. FeCl_{2 ·} 2H₂O, and Fe₃O₄ in superparamagnetic and ferromagnetic states, as well as Fe_1?x O were formed. The formation of a superparamagnetic phase of Fe₃O₄ within the “spike” regions was verified by low temperature measurements. The effects observed were interpreted in terms of the “thermal spike” model. The calculated temperatures and radii of “spikes” formed by iron, chloride and oxygen ions are in good agreement with observation for superparamagnetic Fe₃O₄.     

Hematite thin films: growth and characterization

We have grown hematite (α–Fe ₂ O ₃) thin films on stainless steel and (001)-silicon single-crystal substrates by RF magnetron sputtering process in argon atmosphere at substrate temperatures from 400 to 800°C. Conversion Electron Mössbauer (CEM) spectra of the sample grown on stainless steel at 400°C exhibit values for hyperfine parameter characteristic of bulk hematite phase in the weak ferromagnetic state. Also, the relative line intensity ratio suggests that the magnetization vector of the polycrystalline film is aligned preferentially parallel to the surface. The X-ray diffraction (XRD) pattern of the polycrystalline thin film grown on steel substrates also corresponds to α–Fe ₂ O ₃. The samples were also analyzed by Atomic Force Microscopy (AFM), those grown on stainless steel reveal a morphology consisting of columnar grains with random orientation, given the inhomogeneity of the substrate surface.     

Mössbauer spectroscopic study on soda-vanadate glass system containing iron

Room temperature Mössbauer spectra of xFe₂O₃: (1-x) (Na₂O: 2V₂O₅) glass system showed that Fe³+ are present at both tetrahedral-“network modifying” and octahedral-“network modifying” sites. A Zeeman hyperfine splitting is also observed for x=0.225 along with a doublet, whose internal magnetic field (?518±5 KOe) and isomer shift are found comparable to that of α-Fe₂O₃.     

Structural and energetic properties of alkylfluoride–BF3 complexes in the gas phase and condensed‐phase media: computations and matrix infrared spectroscopy

We have undertaken an experimental and computational study of the structural properties of a few alkylfluoride–BF₃ complexes (RF′–BF₃), which are proposed intermediates in a certain class of Friedel–Crafts reactions. Using density functional theory and second‐order Møller–Plesset calculations, we have obtained gas‐phase structures, frequencies, and B–F′ bond potentials for CH₃F–BF₃, (CH₃)₂CHF–BF₃, and (CH₃)₃CF–BF₃. All the complexes are weakly‐bonded in the gas phase, with B–F′ distances (X3LYP/aug‐cc‐pVTZ) of about 2.4 Å and binding energies (MP2/aug‐cc‐pVTZ) ranging from 5.4 and 6.7 kcal/mol. Accordingly, gas‐phase bond potentials are relatively shallow and flat for these complexes. However, even though the inner walls of the potentials are rather soft (the energies rise by only about 5 to 10 kcal/mol between 2.4 and 1.6 Å), we observe no global or local minima at short B–F′ distances. For the (CH₃)₂CHF–BF₃ and (CH₃)₃CF–BF₃ potentials in dielectric media, we do observe a distinct flattening along the inner wall, which results in shelf‐like region near 1.7 Å, but this feature is not a true local minimum. We have also obtained low‐temperature infrared spectra of the (CH₃)₂CHF–BF₃ complex in solid neon, and the frequencies agree quite favorably with those obtained via computations, which validates the computational assessment of the gas‐phase complexes. Copyright © 2011 John Wiley & Sons, Ltd.     

ESR study of Fe3+ and Mn2+ ions on trigonal sites in ilmenite structure MgTiO3

Electron spin resonance has been observed for Fe³⁺ and Mn²⁺ ions occupying sites with trigonal symmetry in undoped and doped Verneuil-grown crystals of the ilmenite type compound MgTiO₃. At 300 K, the fine structure parameters in the spin Hamiltonian are (in 10^?4cm^?1) D = +844 (± 1), (a? F) = +118 (± 1), a = 69 (± 7) for Fe³⁺ and D = +164 (± 1), (a ? F) = +10.2 (± l), a = 7.0 (± 1) for Mn²⁺. These values are compared with literature data for Fe³⁺ and Mn²⁺ in other oxides, especially Al₂o₃, with particular reference to the recent “superposition” theory of the effect of a trigonal distortion. From the orientation of the axes of cubic pseudosymmetry of the spin Hamiltonian, and with the assumption that a has the same sign for both ions, it is proposed that Fe³⁺ and Mn²⁺ occupy the same octahedral site, namely the Mg²⁺ site. Anomalous line splittings observed for one sample were attributed to twinning on (0001) or {1120} planes.     

Comparative analysis of the twin and heterophase structures in (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 crystals

The specific features of the coexistence of phases in heavily twinned crystals of the (1?x)Pb(Mg_1/3Nb_2/3)O₃?xPbTiO₃ system in the vicinity of the morphotropic phase boundary (0.30≤x≤0.40) are investigated. The phase transformations in crystals at x = const during cooling are considered at electric field strengths E=0 and 0.1 MV/m. The conditions of the formation of interphase boundaries (zero net strain planes) are determined for different first-order phase transitions. The results of calculating the tetragonal-monoclinic M _C and monoclinic-monoclinic (M _C –M _A ) phase transitions are represented in the form of “twin state-interphase boundary” diagrams. The effect of a 90° domain (twin) structure of the tetragonal phase on the heterophase state associated with the presence of monoclinic phases is analyzed.