Occupation Of Octahedral Sites Research Articles

Formation mechanism of the Nb2C phase in the Nb–1Zr–0.1C (wt.%) alloy and interrelation between γ, β and α-Nb2C carbide phases

In the present investigation, the formation of γ-Nb2C in the Nb–Zr–C alloy and its interrelation with other Nb2C carbide phases (α, β) has been studied. The use of synchrotron X-ray diffraction experiments, crystallographic analysis, high resolution imaging, as well as the determination of the relevant orientation relationship, between the second phase and matrix, has led to a proposed mechanism of γ-Nb2C. It has been shown that the transformation of Nb to γ-Nb2C takes place by the occupation of octahedral sites in the bcc Nb lattice by carbon atoms. The mechanism proposes minimal movement of Nb atoms during formation of γ-Nb2C. Further, it has been shown that the sequence of phase transformations (γ-Nb2C → β-Nb2C → α-Nb2C) involves ordering of vacancies whereas the basic lattice in all these three structures remains essentially same. First principle calculations aimed at calculating the formation energies of these carbides have revealed that the difference in these energies to be rather low. This rationalized the observation of a metastable disordered structural phase (γ-Nb2C) in the present study. It has been shown that the arrangement of the carbon atoms in the γ-Nb2C phase leads to the accumulation of strain along the [0001] direction which is accommodated by the creation of lattice defects. This strain destabilizes the γ-Nb2C phase and leading to its dissolution at temperatures as low as 800 °C.

Sourcing limestone masonry for restoration of historic buildings, a spectroscopic pilot study

This study presents a combined Fourier transform (FT) mid-infrared, laser Raman and Commission internationale d’eclairage (CIE) L*a*b*system analysis of quarry-derived impure limestone and fallen masonry from a medieval listed building situated in the south east of England, to ascertain how spectroscopic information can be collectively employed to identify the most exacting possible replacement stone source.Data shows that subtle differences in [Al] and [Fe3+] octahedral and tetrahedral site occupancy in glauconite group clays registered in the mid-infrared [3530 cm−1/3620 cm−1] absorption ratio exerts some influence on L*Cab*hab*values. Increases in L*and Cabare associated with decreasing clay content. Theoverall weakness of correlations between infrared and visible range spectral attributes indicates multiple contributing sources to overall color. Evidence indicates that the degree of laser Raman induced background noise is related to the overall calcite content and that activators of fluorescence at 785 nmexcitation wave length may also contribute to rock color. The results are utilized to define closest matching quarry samples to the fallen masonry.

Investigation of Cationic Distribution in Zn1−x Ni x Fe2O4 (x = 0.0 to 1.0) by Employing X-ray Absorption Fine Structure Spectroscopy

In the present study, x-ray absorption fine structure (XAFS) spectroscopy has been employed to investigate the cationic distribution in Zn1−xNixFe2O4 (x = 0.0 to 1.0) spinel ferrites. The phase purity and lattice parameters of freshly synthesized Zn1−xNixFe2O4 were verified by laboratory x-ray diffraction analysis. It is observed that lattice parameters decrease with increase in the Ni concentration, while the system retains cubic symmetry in Fd3m space group. To investigate the local structural environment and cationic distribution, XAFS data were collected at room temperature for the Fe, Zn, and Ni K-edges in Zn1−xNixFe2O4, using the x-ray absorption facilities of the Elettra synchrotron source, Trieste, Italy. In the case of Fe, pre-edge features provide valuable information regarding the octahedral or tetrahedral site occupancy, while Zn and Ni edges do not show any appreciable change in the near-edge region. From extended x-ray absorption fine structure (EXAFS) spectroscopy, it is observed that all Zn and Ni atoms occupy tetrahedral and octahedral sites, respectively, whereas Fe atoms occupy octahedral sites in ZnFe2O4 but for the other compositions are present at both octahedral and tetrahedral sites. The occupancy of Fe at octahedral sites decreases with increasing Ni concentration in Zn1−xNixFe2O4. These results demonstrate that the local environment of Zn does not show significant variations as the changes at second nearest neighbors are due to Fe and Ni, which have similar scattering factors. However, variations in the Ni surroundings are due to the change of Fe and Zn ratio at A sites, having a relatively larger difference in scattering factor. However, the changes observed in the EXAFS spectra of Fe are a consequence of variation in the occupancy of Fe at tetrahedral and octahedral sites.

Magnetocrystalline anisotropy of NiZnFe2O4

A recently proposed model, based on the relative occupancy of tetrahedral and octahedral sites by different cations, was used to study the magnetocrystalline anisotropy of mixed Ni–Zn ferrite nanoparticles. According to this model, the total magnetocrystalline anisotropy is the weighted average of the contributions of the anisotropies of Fe3+ and M2+ ions in A and B sites. The model predictions are confirmed in the case of nickel–zinc ferrite.

Structural and magnetic evaluation of substituted NiZnFe2O4 particles synthesized by conventional sol–gel method

The aim of this study is to evaluate the structural and magnetic properties of Ni–Zn doped ferrite with trivalent Al3+ and Cr3+ cations substitution in Ni0.6Zn0.4Fe2−xCrx/2Alx/2O4 (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) synthesized by employing conventional sol–gel method. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM), Mössbauer spectroscopy (MS) and vibrating sample magnetometer (VSM) analysis were carried out in order to characterize the structural and magnetic properties of particles. The XRD results confirmed the formation of single phase of spinel ferrite particles for a whole series of samples. The results of FTIR analysis indicated that the functional groups of Ni–Zn spinel ferrite were formed during the sol–gel process. Furthermore, FE-SEM micrographs revealed that the distribution of particles size is narrow. According to Mössbauer spectra,the doped cations are replaced in iron site occupancy of octahedral sites. It was found that with an increase in substitution contents magnetization decreased due to occupation of Al and Cr cations at low level substitutions in octahedral sites.

Thermal Evolution of Cation Distribution/Crystallite Size and Their Correlation with the Magnetic State of Yb-Substituted Zinc Ferrite Nanoparticles

Evolution of the structural and magnetic properties of ZnFe1.95Yb0.05O4 nanoparticles, prepared via a high-energy ball milling route and exposed to further thermal annealing/heating, was assessed in detail and correlation of these properties explored. While as-prepared spinel nanoparticles possess a high degree of inversion, heating of the sample to ∼500 °C is found to rapidly alter the cation distribution from mixed to normal, in agreement with the known cation preferences. Under the same conditions the crystallite size only slowly grows. By further thermal treatment at higher temperatures, the crystallite size is changed more appreciably. An interrelationship among the lattice parameter, octahedral site occupancy, and crystallite size has been established. The observations are (a) both the site occupancy of Fe3+ at octahedral 16d spinel sites (N16d(Fe3+)) and the cubic lattice parameter rapidly increase with an initial increase of the crystallite size, (b) the lattice parameter increases with increasing occupancy, N16d(Fe3+), and (c) there appears to be a critical nanoparticle diameter (approximately 15 nm) above which both the site occupancy and lattice parameter values are saturated. The magnetic behavior of the annealed samples appears to be correlated to the evolution of both the cation distribution and crystallite size, as follows. As-prepared samples and those annealed at lower temperatures show superparamagnetic behavior at room temperature, presumably as a consequence of the Fe3+ distribution and strong Fe3+(8a)–O–Fe3+(16d) superexchange interactions. Samples with a nanoparticle diameter greater than 12 nm and with almost normal distributions exhibit the paramagnetic state. The coercive field is found to decrease with an increase of the crystallite size. Partial Yb3+/Fe3+ substitution is found to increase the inversion parameter and saturation magnetization. Detailed knowledge of the thermal evolution of structural/microstructural parameters allows control over the cation distribution and crystallite size and hence the magnetic properties of nanoferrites.

Is a cation ordering transition of the Mg‐Fe olivine phase in the mantle responsible for the shallow mantle seismic discontinuity beneath the Indian Craton?

We use first‐principles molecular dynamics simulations to study the behavior of cation ordering in the non‐equivalent octahedral sites of Mg‐Fe olivine solid solutions. Our theoretical calculations confirm the previous experimental finding that Mg2+ and Fe2+ can invert their octahedral site occupancy at a critical temperature. Assuming that the site preference of Fe changes discontinuously between two states in which it is completely restricted to either M1 or M2 sites, we have calculated the transition temperature, Tt, between the two extreme states. Under ambient pressure Tt is calculated to be 520°C that agrees fairly with the experimental finding in which, however, the ordering state changed discontinuously over a much smaller range of the site occupancy of Fe. Tt is found to be pressure sensitive, showing an increase by 30 to 100°C per unit GPa, depending upon the iron content. Using the Indian continental geotherm, we estimate a depth of around 75 Km corresponding to the calculated transition pressure and temperature of cation ordering, which matches well with the depth for the Hales discontinuity marked by a jump of shear wave velocity by ∼4%. For olivine solid solutions with 12.5% iron, the ordering transition increases Vs from 4.5 to 4.7 Km/s. Both the inferences, viz. depth of discontinuity and magnitude of velocity increase find support from the modeling of teleseismic earthquake waveforms recorded over broadband seismographs on the Dharwar Craton. This leads us to infer that the cation ordering transition in ferromagnesian olivine might be a potential factor for the Hales discontinuity.

<sup>1</sup>H NMR Study of Hydrogen Site Occupancy in Hydrides of Disordered Ti-V and Ti-V-Cr Alloys

According to 1H NMR study in fcc hydrides of disordered Ti-V alloys hydrogen atoms are distributed over both octahedral and tetrahedral sites. The occupancy of octahedral sites increases with the vanadium concentration. In hydrides of ternary Ti-V-Cr alloys hydrogen atoms occupy tetrahedral sites only.

Study of NiFe2O4 nanoparticles using Mössbauer spectroscopy with a high velocity resolution

The nanocrystalline NiFe2O4 particles prepared by solution combustion synthesis technique using different fuels such as ethylene-diamine-tetra-acetic acid (NA sample) and urea (NB sample) were studied using magnetic measurement and 57Fe Mössbauer spectroscopy with a high velocity resolution. The temperature dependence of magnetization is different for the two samples. Mössbauer spectra demonstrate the necessity to use more than two magnetic sextets, usually used to fit the NiFe2O4 nanoparticles spectra. Evaluation of the different local microenvironments for Fe in both tetrahedral (A) and octahedral (B) sites, caused by different Ni2 + occupation of octahedral sites, demonstrates at least five different local microenvironments for both A and B sites. Therefore, the Mössbauer spectra were fitted by using ten magnetic sextets which are related to the spread 57Fe location in octahedral and tetrahedral sites.

A simple model for the magnetocrystalline anisotropy in mixed ferrite nanoparticles

A simple model, based on the relative occupancy of tetrahedral and octahedral sites by different cations, is proposed for the magnetocrystalline anisotropy of mixed ferrite nanoparticles. According to this model, the total magnetocrystalline anisotropy is the weighted average of the contributions of the anisotropies of Fe3+ and M2+ ions in A and B sites. The model predictions are confirmed in the case of cobalt-zinc ferrite.

Chemical composition and species attribution of tourmalines from a rare-metal pegmatite vein with scapolite, Sangilen Upland, Tuva

A detailed study of the chemical composition and substitutions in calcium tourmalines from a scapolite-bearing rare-metal pegmatite vein from the Sol’bel’der River basin has shown that their species attribution is determined by occupancy of octahedral site Y. The composition of the yellow tourmaline most abundant in the central part of the pegmatite bodyis rather constant and characterized by the ideal formula Ca(Mg2Li)Al6(Si6O18)(BO3)3(OH)3F. Variations in the chemical composition of zonal tourmaline crystals from the contact part of the pegmatite are controlled by abrupt change in the chemical medium during their formation. The yellow cores of these crystals are close in composition to tourmaline from the central part of the pegmatite vein. The Mg content abruptly decreases toward the crystal margin: Mg2+ → Fe2+, 2Mg2+ → Li+ + Al3+, and Mg2+ + OH → Al3+ + O2−. The composition of dark green marginal zones in tourmaline is characterized by the ideal formula Ca(Al1.5Li1.5)Al6(Si6O18)(BO3)3 (OH2O)(F). The results indicate specific formation conditions of pegmatite. The crystallochemical formulas of the studied tourmalines allow us to regard them as new mineral species in the tourmaline group.

Epitaxial transformation of hcp–fcc Ti sublattices during nitriding processes of evaporated-Ti thin films due to nitrogen-implantation

Abstract Atomistic transformation processes of Ti films due to N-implantation have been clarified through in-situ observations by using transmission electron microscope (TEM) along with molecular orbital calculations. The N 2 + ions with 62 keV are implanted into as-deposited Ti films which consist of hcp-Ti and TiH x with preferred orientations, in the 400 kV analytic high resolution TEM combined with ion accelerators. Thus, titanium nitride (TiN y ) films with preferred orientations are epitaxially formed by the inheritance of partial atomic arrangement of hcp-Ti or TiH x in as-deposited Ti films and by the occupation of octahedral sites by N atoms, which elucidates that epitaxial transformation of hcp–fcc Ti sublattices occurs. The analysis of electronic structure of Ti films during the implantation clarifies that octahedral sites of hcp-Ti with larger space have lower electron density, which leads to the invasion of N ions into octahedral sites. Thus, the hcp–fcc transformation is induced by the shear in the direction on the (0 0 · 1) plane, promoted by the forming of covalent bonds mainly composed of hybridized orbitals due to combination of Ti3d and N2p orbitals, and by the weakening of Ti–Ti bonds.

First-principles study of the solid solution of hydrogen in lanthanum

Results from first-principles investigations of the energetical, structural, electronic, and vibrational properties of model structures probing the metal-rich region of the lanthanum-hydrogen system, i.e., the region of the solid solution of hydrogen in lanthanum, are presented. We have studied the site preference and the ordering tendency of hydrogen atoms interstitially bonded in close-packed lanthanum. Spatially separated hydrogen atoms have turned out to exhibit an energetical preference for the occupation of octahedral interstitial sites at low temperature. Indications for a reversal of the site preference in favor of the occupation of tetrahedral interstitial sites at elevated temperature have been found. Linear arrangements consisting of pairs of octahedrally and/or tetrahedrally coordinated hydrogen atoms collinearly bonded to a central lanthanum atom have turned out to be energetically favorable structure elements. Further stabilization is achieved if such hydrogen pairs are in turn linked together so that extended chains of La-H bonds are formed. Pair formation and chain linking counteract the energetical preference for octahedral coordination observed for separated hydrogen atoms.

Black and green pigments based on chromium–cobalt spinels

Chromium and cobalt oxides are widely used in the manufacture of industrial pigments. In this work, the Co(Co 2− x Cr x )O 4 powders with different chromium concentrations ( x = 0, 0.25 and 1) were synthesized by the polymeric precursor method, heat treatment between 600 and 1000 °C. These powders were characterized by X-ray diffraction, infrared spectroscopy, colorimetry, UV–vis absorption and X-ray photoelectron spectroscopies. Even with the addition of chromium, the XRD patterns revealed that all powders crystallize in a single spinel cubic structure. The spinels with higher cobalt amount, Co(CoCr)O 4 and Co(Co 1.75Cr 0.25)O 4, displayed a dark color, without the Co 3+ reduction observed in Co 3O 4 between 900 and 950 °C. The spinel with higher chromium amount, CoCr 2O 4, was green. The colors were directly related to the occupation of tetrahedral and octahedral sites by the chromophores, as well as to the different oxidation states of chromium and cobalt. The different optical band gap values estimated from UV–vis spectra suggested the existence of intermediary energy levels within the band gap. X-ray photoelectron spectroscopy confirmed an increasing presence of Co(III) and a decreasing amount of Cr(VI) with cobalt enrichment.

Catalytic activities of cobalt, nickel and copper ferrospinels for sulfuric acid decomposition: The high temperature step in the sulfur based thermochemical water splitting cycles

The catalytic decomposition of sulfuric acid is the most endothermic step of the sulfur based water splitting thermochemical cycles, which are promising technologies for large scale hydrogen production in future. In the present study the catalytic activities of three ferrospinels AFe 2O 4 (A = Co, Ni, Cu) were evaluated for high temperature sulfuric acid decomposition reaction. Catalyst characterization by Mössbauer spectroscopy confirmed the occupancy of octahedral and tetrahedral sites by Fe 3+ ions in all three inverse spinels. The temperature programmed reduction studies revealed that the reducibility of Fe 3+ was greatly enhanced in CuFe 2O 4 as compared to other ferrites. Copper ferrite was found to be the most active catalyst for the reaction with ∼78% conversion at 800 °C. Presence of sulfate species on the spent catalysts was revealed by an ex situ analysis of the spent catalyst samples by FTIR, SEM and evolved gas analysis (EGA). FTIR spectra of all the three spent catalyst samples exhibit four prominent peaks in the region 950–1200 cm −1, which is an indicative of C 2v symmetry and bidentate sulfate coordination. A plausible mechanism for the sulfuric acid decomposition over spinel ferrites was proposed via the metal sulfate formation and then decomposition followed with an oxygen evolution step with the sulfate decomposition being the rate determining step at higher temperatures. EGA showed evolution of SO 2 as a decomposition product from existed sulfate of spent catalysts at high temperatures, with the rate of SO 2 evolution following the order: CuFe 2O 4 > NiFe 2O 4 > CoFe 2O 4. The enhanced rate of decomposition of the sulfates of copper ferrite can be attributed to the higher electronegativity of Cu 2+ as compared to Ni 2+ and Co 2+, which renders the S–O bond in the mixed metal sulfate weaker than others and thus more susceptible to dissociation. The lower thermal stability of sulfate and better reducibility are responsible for the improved catalytic properties of copper ferrite among the three ferrospinels investigated for sulfuric acid decomposition.

Synthesis characterization and magnetic properties of Cr-substituted NiCuZn nanocrystalline ferrite

Egg-white method was used to produce nanocrystalline Cr-substituted NiCuZn ferrites; Ni 0.50Cu 0.25Zn 0.25Fe 2− x Cr x O 4 (0.0 ≤ x ≤ 1.0) from stoichiometric mixture of their respective metal nitrate. The structural, morphological and magnetic properties of the products were determined by X-ray powder diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The average crystallite sizes obtained from XRD were between 15 and 25 nm. Magnetization measurements indicated that when the Cr substitution increases the saturation magnetization decreases due to the magnetic character of the chromium ions which prefers octahedral site occupation. The coercivity was found to change proportionally with the particle sizes of the investigated ferrites.

Phase transformation in an yttrium–hydrogen system studied by TEM

Phase transformations in Pd-capped epitaxial yttrium films grown on (0 0 0 1) sapphire substrates covered with a Ti buffer layer and hydrogenated for different times were studied using transmission electron microscopy (TEM). For short hydrogen charging times, the phase transformation from α-Y to β-YH 2 is associated with the nucleation and growth of two orientational variants, which after coalescence form twin-related lamellae of the β-YH 2 phase with twin interfaces parallel to the substrate plane. Shockley partial dislocations are present at the twin boundaries; their glides during phase transformation are responsible for the formation of the twin lamellae. Superlattice reflections were observed for β-YH 2, and the existence of a new long-range ordered superstoichiometric YH 2+ x phase was suggested. A structural model of the ordering based on the occupation of octahedral interstitial sites by H in a doubled cell of Y-face-centered cubic was offered. For samples hydrogenated for longer times, β-YH 2-to-γ-YH 3 phase transformation was accompanied by cracking along the twin boundaries, which eventually developed into a network of pores and caused significant swelling of the films. No γ-YH 3 phase was observed directly in TEM because of its unstable nature under the high vacuum of a microscope column. The fully transformed YH 3 films have over a 60% increase in its thickness, which is mostly accounted for by the high volume fraction of pores.

Structure, Infrared Radiation Properties and Mössbauer Spectroscopic Investigations of Co 0.6Zn 0.4Ni xFe 2- xO 4 Ceramics

Ferrite compound Co 0.6 Zn 0.4 Ni x Fe 2- x O 4 was synthesized by solid state reaction. The structure and performance of Co 0.6 Zn 0.4 Ni x Fe 2- x O 4 compounds were studied by Mossbauer spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared radiant tester. Mossbauer spectroscopy and XPS analysis show the valence states and distribution of cations in Co 0.6 Zn 0.4 Ni x Fe 2- x O 4 . Zn 2+ has invariably shown preference for the tetrahedral sites, and Ni 2+ has the preference for the octahedral sites. The occupancy of Fe 3+ linearly increases on the tetrahedral sites, and sharply decreases on the octahedral sites with increasing x , owing to its gradual replacement by Ni 2+ on the octahedral sites. It indicates that due to the occupation of octahedral sites by the majority of Ni 2+ , Fe 3+ decreasingly migrated from the octahedral sites to the tetrahedral sites and substituted Co 3+ sites, which made the number of Co 3+ in the tetrahedral sites decreasing. According to the measurement results of XRD and the infrared radiant tester analysis, the lattice parameter and infrared radiance have shown a nonlinear variation, exhibiting the infrared average radiance of 0.92 in the 8-14 μm waveband, and the results demonstrate that these Co-Zn-Ni spinel ferrites have potential for application in a wide range of infrared heating and drying materials.

Carbon occupancy of interstitial sites in vanadium carbide films deposited by direct current reactive magnetron sputtering

Vanadium carbide thin films were deposited on Si substrates by direct current reactive magnetron sputtering from a V target in Ar/CH 4 plasma, varying the Ar/CH 4 partial pressure ratio and substrate temperature. The films were characterized by glancing angle X-ray diffraction and Rutherford backscattering spectrometry. Well defined crystalline structures were obtained for CH 4 content higher than 13%. The increase of substrate temperature during deposition diminishes the film thickness slightly while diminishing substantially the C/V atomic ratio. The intensity ratio of the Bragg peaks (111)/(200) decreases for increasing substrate temperature. This result is discussed in terms of a proposed mechanism for interstitial diffusion of carbon atoms in vanadium carbide thin films with fcc-like crystalline structure and the temperature dependence of carbon occupancy of tetrahedral or octahedral interstitial sites.

Structure and magnetic properties of Ca 2Fe 1−xMn xAlO 5+δ

Ca 2Fe 1− x Mn x AlO 5 (0⩽ x⩽1) compounds were prepared by a self-combustion method under air ( x=0, 0.1, 0.2 and 0.3) and nitrogen ( x=0.5, 0.7 and 1.0). The samples prepared under nitrogen were successfully oxidized after short annealing under air. Both X-ray powder diffraction (XRD) Rietveld analysis and electron diffraction revealed that all compounds adopt the brownmillerite-type structure. All samples present an overall antiferromagnetic behaviour and data from magnetic measurements and Mössbauer spectroscopy allowed to conclude that the transition temperature decreases as Mn content increases for x⩽0.3 and increases in the case of the x⩾0.5 compounds. Except for x=1, chemical disorder due to the occupancy of both octahedral and tetrahedral sites by different metals as well as the competition between different moments’ orientation induce a complex magnetic behaviour characterized by magnetic frustration and canted antiferromagnetism. Mössbauer spectroscopy and chemical titrations also allowed to conclude about the preferential oxidation of Mn 3+ over Fe 3+, obtained by thermal treatment under air of the x=0.5 and 0.7 compositions.