Octahedron Sites Research Articles

X-ray photoelectron spectroscopy and low temperature Mössbauer study of Ce3+ substituted MnFe2O4

Spinel Mn2+ Fe 2−x 3+ Ce x 3+ O4 (where x = 0.01–0.05) polycrystalline nanoparticles were fabricated by solution combustion method using glucose and urea as fuels. X-ray diffractogram (XRD) proved the witness of the cubic crystal consort of products which have the crystallite sizes in between 12 and 8 nm. The size of the sample was accomplished via transmission electron microscopy. The elemental composition of iron and manganese ions valences of MnCexFe2−xO4 samples were analyzed by X-ray photoelectron spectroscopy. XPS data reveals iron in all samples was trivalent; manganese was divalent, although tetravalent manganese ions were present on the surface of particles. Magnetic characterization of samples was done via Mossbauer spectroscopy at room temperature and at low temperatures (15 K). The Mossbauer analysis determines the consequence of Ce3+ substitution on isomer shift and quadrupole splitting of all samples. The low temperature Mossbauer spectroscopy results indicate that the presence of Ce3+ ions in the octahedron sites causes the decrease in the concentration of Fe3+ ions.

Ce/Mn dual-doped LaAlO3 ceramics with enhanced far-infrared emission capability synthesized via a facile microwave sintering method

In this work, Ce/Mn dual-doped LaAlO3 ceramics have been synthesized via a facile microwave sintering method. From the detailed investigations of Fourier transform infrared spectroscopy, it has been found that Ce/Mn co-doping can dramatically improve the far-infrared emission capability of LaAlO3 ceramics in the range of 8–14 μm. After co-doping of Ce and Mn, Ce4+ ions in polyhedron site (A-site) induce the transformation of Mn4+ to Mn2+ in octahedron site (B-site), which causes the severe lattice distortion of the unit cell and strengthens the vibration intensities of asymmetrical Mn-O-Mn and Mn-O-Al bonds for the promotion of the far-infrared emissivity. Such dual-doped and perovskite-type LaAlO3 ceramics with excellent far-infrared emission shows great potential as a new generation of far-infrared radiation materials for energy conservation applications.

Effect of Ce3+ Ion on Structural and Hyperfine Interaction Studies of Co0.5Ni0.5Fe2−xCexO4 Ferrites: Useful for Permanent Magnet Applications

Nanoparticles of Co0.5Ni0.5Fe2−xCexO4 (where x = 0.0, 0.01, 0.015 and 0.02) ferrites are prepared by the modified solution combustion method using a mixture of fuels and are characterized to understand their structural, microstructural and magnetic properties. The X-ray diffraction is used to confirm the formation of a single-phase cubic spinel structure. The average crystallite sizes are calculated using the Scherrer formula and are found to be less than 50 nm. The microstructural features are obtained by the scanning electron microscopy, and the compositional analysis is done by using the energy-dispersive spectroscopy. The transmission electron microscopy (TEM) investigations show that the synthesized ferrites are made up of very fine spherical nanoparticles. The influence of a rare-earth element (Ce3+) on the magnetic properties of the samples was studied using the Mossbauer spectroscopy. The Mossbauer spectroscopy reveals the formation of broadened Zeeman lines and quadrupole-split lines and the presence of the Fe3+ charge state at B sites in the samples. The quadrupole splitting shows that the orientation of the magnetic hyperfine field with respect to the principle axes of the electric field gradient was random. The magnetic hyperfine field values indicate that the A sites have more A-O-B superexchange interactions than the B sites. The coexistence of magnetic sextet and a doublet component on the room-temperature spectra suggests superparamagnetic properties of the nanoparticles. The low-temperature (15 K) Mossbauer spectroscopy explores the paramagnetic relaxation in the nanoparticles. The area under the sextet refers to Fe3+ concentrations in the tetrahedral and octahedral sites of the ferrite. This study confirms that the Ce3+ substitution of Fe3+ only for octahedron sites causes the decrease in Fe-O-Fe arrangement. The effect of Ce3+ doping on the magnetic properties of Co0.5Ni0.5Fe2O4 is examined from the vibrating sample magnetometry (VSM) spectra. Saturation magnetization values are decreased initially and then increased, as result of Ce3+ doping. This can be explained by Neel’s two-sub-lattice model. Further, the value of coercivity is found to be increasing with increasing Ce3+ concentration. The obtained results of M-H loop with improved coercivity (from 851 to 1039 Oe) by Ce3+ doping of Co0.5Ni0.5Fe2O4 demonstrate the usefulness for permanent magnet applications.

Influence of inverse spinel structured CuGa2O4 on microwave dielectric properties of normal spinel ZnGa2O4 ceramics

AbstractSpinel Zn1‐xCuxGa2O4 (x = 0‐0.15) ceramics were prepared by the conventional solid‐state method. Only a single phase was indexed in all samples. The continuous lattice contraction of ZnGa2O4 unit cell was caused by Cu2+ substitution, and the lattice parameter shows a linear correlation with the content of Cu. The refined crystal structure parameters suggest that Cu2+ preferentially occupies the octahedron site, and the degree of inversion of Zn1‐xCuxGa2O4 (x = 0‐0.15) ceramics almost equals to the content of Cu2+. The relative intensity of A*1g mode in Raman spectra confirm that the degree of inversion climbed with the growing content of Cu2+. The experimental and theoretical dielectric constant of Zn1‐xCuxGa2O4 ceramics fit well. Zn1‐xCuxGa2O4 (x = 0.01) ceramics sintered at 1400°C for 2 h exhibited good microwave dielectric properties, with εr = 9.88, Q × f = 131,445 GHz, tanδ = 6.85 × 10−5, and τf = −60 ppm/°C.

Cation distributions and microwave dielectric properties of Cu-substituted ZnGa2O4 spinel ceramics

Abstract Spinel Zn1-xCuxGa2O4 (x = 0–0.05) ceramics were prepared by the conventional solid-state method. Only a single phase was indexed in all samples. The relative density increased with Cu-substitution. Refined crystal structure parameters suggested that Cu2+ preferentially occupies the octahedron site and formed inverse spinel structure. The relative intensity of A*1g mode in Raman spectra confirmed that the inversion degree climbed with the growing content of Cu. The differences of ion dielectric polarizability resulted in the shift of absorption bands in Fourier transform infrared (FT-IR) spectroscopy. Ion dielectric polarizability and cell volumes differences affected permittivity (er) and increased er slightly during Cu-substitution. The temperature coefficient of resonant frequency (τf) kept steady, however, the quality factor (Q × f) value of Zn1-xCuxGa2O4 ceramics increased by 50% due to the Cu-substitution (from 85,824 GHz to 131,445 GHz). Zn0.99Cu0.01Ga2O4 ceramic s sintered at 1400 °C for 2 h exhibited good microwave dielectric properties, with er = 9.88, Q × f = 131,445 GHz, tan δ = 6.85 × 10−5,and τf = −60 ppm/°C.

F−-Eu3+ charge transfer energy and local crystal environment in Eu3+ doped calcium fluoride

A series of CaF2:xEu3+ (x = 0.01–0.05) phosphors were synthesized by coprecipitation and high temperature solid-state method. Three different F--Eu3+ charge transfer (CT) bands in CaF2:xEu3+ (x = 0.01–0.05) were detected through the PL spectra. Three local crystal environments (deformed Oh octahedron, C3V, C4V) were proposed in Eu3+ doped CaF2. The broad peak at 277 nm is F- → Eu3+ CT peak, in which Eu3+ ions located deformed Oh octahedron sites, the broad peak at 312 nm is due to Eu3+ ions situating at C4v sites, also, Eu3+ ions with C3v sites can make the F- → Eu3+ CT produce a 38-nm redshift to 315 nm. Density functional theory (DFT) was performed to calculate the energy band gap(Eg) according to the three models. The calculation results consist with the experiment. The excitation peaks can be tuned in the same host through changing the local structure of Eu3+.

Low temperature Mössbauer spectroscopic studies on Sm3+ doped Zn-Mn ferrites

For the first time, we report on the low temperature Mössbauer spectroscopic study of Zn2+0.5Mn2+0.5Sm3+xFe3+2−xO4 (where x=0.01–0.05) prepared by the modified solution combustion method using a mixture of urea and glucose as a fuel. The Mössbauer spectroscopy at room and low temperatures was applied to understand the magnetic properties of the samples. The room temperature Mössbauer spectroscopy results suggest that the occupation of the octahedral sites by Sm3+ ions leads to the distortion enhancement of 57Fe nuclei environments, which leads to an increase in quadrupole splitting Δ values of D2 and D3 doublets. The low temperature Mössbauer spectroscopy results indicate that the presence of Sm3+ ions in the octahedron sites causes the decrease in the number of Fe–O–Fe chains. The transformation of Mössbauer spectra doublets into Zeeman sextets is accompanied by a significant decrease in the magnitude IM of Mössbauer spectra intensity within the 0–1.2mm/s velocity range normalized to its value at 300K. This drop in the temperature dependence of IM allows one to obtain the magnetic phase transition temperature TM from the Mössbauer experiment.

Spin state of Co3+ ions in EuBaCo2–x O5.5–δ in the paramagnetic temperature range

The magnetic studies of EuBaCo2O5.50, EuBaCo1.93O5.40, and EuBaCo1.90O5.36 have been performed. Layered cobaltites EuBaCo2–x O5.5–δ demonstrated the ferrimagnetic ordering in the temperature range 200 K < T < 650 K. The ferromagnetic Curie temperature T C almost did not change as the number of Co3+–O–Co3+ bonds decreased. The contributions of the ferro- and antiferromagnetic interactions were nearly the same and determine the relationship T C > T N and T C < T N. At T > T C, the antiferromagnetic order was retained in the EuBaCo1.90O5.36. As temperature increased, EuBaCo2–xO5.5–δ conserved the IS state in pyramids, and gradual LS → IS → HS transitions occurred in octahedron sites. The increase in the effective magnetic moment μeff in EuBaCo1.90O5.36 at T > 580 K demonstrated the transition of Co3+ ions to the HS state at lower temperatures as compared to the EuBaCo2O5.50 stoichiometric composition, and the structural distortions and the increase in the unit cell volume favored to this transition.

Siderophore-Mediated Iron Dissolution from Nontronites Is Controlled by Mineral Cristallochemistry.

Bacteria living in oxic environments experience iron deficiency due to limited solubility and slow dissolution kinetics of iron-bearing minerals. To cope with iron deprivation, aerobic bacteria have evolved various strategies, including release of siderophores or other organic acids that scavenge external Fe(III) and deliver it to the cells. This research investigated the role of siderophores produced by Pseudomonas aeruginosa in the acquisition of Fe(III) from two iron-bearing colloidal nontronites (NAu-1 and NAu-2), comparing differences in bioavailability related with site occupancy and distribution of Fe(III) in the two lattices. To avoid both the direct contact of the mineral colloids with the bacterial cells and the uncontrolled particle aggregation, nontronite suspensions were homogenously dispersed in a porous silica gel before the dissolution experiments. A multiparametric approach coupling UV-vis spectroscopy and spectral decomposition algorithm was implemented to monitor simultaneously the solubilisation of Fe and the production of pyoverdine in microplate-based batch experiments. Both nontronites released Fe in a particle concentration-dependent manner when incubated with the wild-type P. aeruginosa strain, however iron released from NAu-2 was substantially greater than from NAu-1. The profile of organic acids produced in both cases was similar and may not account for the difference in the iron dissolution efficiency. In contrast, a pyoverdine-deficient mutant was unable to mobilize Fe(III) from either nontronite, whereas iron dissolution occurred in abiotic experiments conducted with purified pyoverdine. Overall, our data provide evidence that P. aeruginosa indirectly mobilize Fe from nontronites primarily through the production of pyoverdine. The structural Fe present on the edges of NAu-2 rather than NAu-1 particles appears to be more bio-accessible, indicating that the distribution of Fe, in the tetrahedron and/or in the octahedron sites, governs the solubilisation process. Furthermore, we also revealed that P. aeruginosa could acquire iron when in direct contact with mineral particles in a siderophore-independent manner.

Direction-dependent RBS channelling studies in ion implanted LiNbO3

Damage formation in ion implanted LiNbO3 was studied by Rutherford backscattering spectrometry (RBS) along various directions of the LiNbO3 crystal. From the results obtained it can be unambiguously concluded that Nb atoms being displaced during ion implantation preferably occupy the free octahedron sites of the LiNbO3 lattice structure and most likely also form NbLi antisite defects.

The investigation of giant magnetic moment in ultrathin Fe3O4 films

The magnetic and transport properties of Fe3O4 films with a series of thicknesses are investigated. For the films with thickness below 15 nm, the saturation magnetization (Ms) increases and the coercivity decreases with the decrease in films’ thickness. The Ms of 3 nm Fe3O4 film is dramatically increased to 1017 emu/cm3. As for films’ thickness more than 15 nm, Ms is tending to be close to the Fe3O4 bulk value. Furthermore, the Verwey transition temperature (Tv) is visible for all the films, but suppressed for 3 nm film. We also find that the ρ of 3 nm film is the highest of all the films. The suppressed Tv and high ρ may be related to the islands morphology in 3 nm film. To study the structure, magnetic, and transport properties of the Fe3O4 films, we propose that the giant magnetic moment most likely comes from the spin of Fe ions in the tetrahedron site switching parallel to the Fe ions in the octahedron site at the surface, interface, and grain boundaries. The above results are of great significance and also provide a promising future for either device applications or fundamental research.

Cation Distribution and Temperature Dependence of Brillouin Function for Nickel-Substituted Manganese–Zinc Ferrites

Ni-substituted manganese–zinc (MnZn) ferrites with the composition of Mn0.506– x Zn0.244Ni x Fe2.250O4.0 ( $x = 0.066{\sim}0.122$ ) have been prepared by the solid-state reaction method. The cation distribution has been investigated by the Rietveld refinement of X-ray diffraction patterns, the microstructure has been observed using a scanning electron microscope, and the magnetic property has been measured using superconductor quantum interference devices and $B$ – $H$ analyzer. The results show that Zn2+ and Ni2+ ions prefer to occupy the tetrahedron site (A sublattice) and octahedron site (B sublattice), respectively. However, Mn2+ and Fe3+ ions can enter into A and B sublattices, where the ratio of Mn2+ ions occupying A and B sublattices is 4:1. The lattice parameter ( $a$ ) of the samples decreases with the increase of Ni-substituted content. Meanwhile, based on the Neel model of collinear-spin ferrimagnetism, the molecular-field coefficients $\omega _{\rm AA}$ , $\omega _{\rm BB}$ , and $\omega _{\rm AB}$ of the Ni-substituted MnZn ferrites have been calculated, and the magnetic moment of A and B sublattices versus temperature $T$ has also been investigated. The fitting results match well with the experimental data. Both $\omega _{\rm AB}$ and $\omega _{\rm BB}$ increase with the increase of the Ni-substituted content, but $\omega _{\rm AA}$ shows the opposite variation trend. The Curie temperature also increases with the increasing of the Ni-substituted content, which is attributed to the enhancement of superexchange interaction for A–B sublattice. In addition, the temperature dependence of initial permeability and core loss has been discussed.

Oxygen vacancy induced magnetization switching in Fe3O4 epitaxial ultrathin films on GaAs(100)

The magnetic and transport properties of half metallic Fe3O4, which are sensitive to the stoichiometry, are the key issue for applications in spintronics. An anomalous enlargement of the saturation magnetic moment is found in a relatively thick sample of epitaxial Fe3O4 film by post-growth oxidation method. The investigation of the thickness dependence of magnetic moment suggests that the enhanced magnetism moment may come from the existence of oxygen vacancies. First-principles calculations reveal that with oxygen vacancies in Fe3O4 crystal the spin of Fe ions in the tetrahedron site near the vacancy is much easier to switch parallel to the Fe ions in the octahedron site by temperature disturbance, supported by the temperature dependence of magnetic moment of Fe3O4 films in experiment.

Oxygen Nonstoichiometry and Electrochemical Properties of LaNiO3-δ

1. Introduction The oxygen reduction process on cathode is large source of resistance in Solid Oxide Fuel Cell (SOFC). To minimize the resistance on cathode, exploring new materials that exhibit fast oxygen reduction process is necessary. LaNiO3-δ is one of the candidates for cathode of SOFC. The Ni in B site of perovskite-type structure form octahedron with the oxygen and control the electronic conduction mechanism. Several works mention that LaNiO3-δ has high performance on the electrical conductivity [1]. However, it suffers from the degradation above 1123 K in air due to the chemical instability. The Ni in the perovskite-type structure is unstable since Ni tends to become Ni2+ instead of Ni3+ [1,2]. Many workers studied the La2NiO4-±for cathode SOFC due to its stability and shows good performance on electrochemical properties. In our study, the stability of LaNiO3-δ was examined with assumption the LaNiO3-δ may be stable in the range of intermediate temperature. In this study, the LaNiO3-δ were examined its stability as function of temperature (773 – 1073 K) by thermogravimetry and coloumetric tritration. Moreover, the electrochemical impedance spectroscopy (EIS) measurement was also performed to see the electrochemical properties of LaNiO3-δ. 2. Experimental LaNiO3-δ was synthesized by the Pechini method and sintered at 1073 K for 6 hours. The single phase of the sample was confirmed by X-ray diffraction. For coulometric titration, the powder of LaNiO3-δwas pressed into a pellet and sintered at 1073 K for 6 hours. An yttria stabilized zirconia (YSZ) tube was used as an electrolyte. The sample powder of 0.5 gram were lumped into the inside of the tube and mechanically crushed and pressed in order to get contact with YSZ. The platinum paste was painted on the outside of the YSZ tube. The measurement was taken by “two-step-forward and one-step-back" to check the reproducibility on the coulometric titration. The oxygen nonstoichiometry was determined by the electric charge which passed through into the cell as described by the equation: Δδ = (C/F)(Ms /ws ) where C, F, Ms , and ws is the total amount of electric charge, Faraday constant, the formula weight of the sample and the weight of the sample, respectively. The oxygen coulometric titration was carried out at different temperatures (773 – 1073 K). Porous LaNiO3-δ was painted into the both sides of Ce0.9Gd0.1O1.95 (GDC) electrolyte. Pt electrodes were used as a reference electrode. Electrochemical properties of the porous LaNiO3-δ electrode were investigated by using impedance spectroscopy in a single chamber cell as functions of temperature (773 – 1073 K) and oxygen partial pressure (10-3-1 bar). In the analyses of the impedance responses, tentative equivalent circuits of R-C elements (a resistance, and a capacitance) were used to identify their physical meanings. 3. Results and Discussion The relative oxygen nonstoichiometry of LaNiO3-δ is shown in the Fig. 1(a). The relative oxygen nonstoichiometry increases with increasing temperature and reducing oxygen partial pressure. The step increase was clearly observed around 10-4 bar at 1073 K, which indicates decomposition due to Ni3+ reduction into Ni2+. The EIS results on LaNiO3-δ porous electrode showed polarization on the low frequency arc. The low frequency arc was assumed to associate with the oxygen reduction process at the electrode. As shown in Fig. 1(b), the area specific conductivity (ASC) of the LaNiO3-δ as a function of temperature and oxygen partial pressure. In our previous works on Ni based electrode (La(Ni,Fe)O3 and La(Ni,Co)O3), the LaNiO3-δ shows better performance on ASC. Even though, the measurement successfully achieved, but the secondary phase may appear during the measurement of oxygen nonstoichiometry and electrochemical properties. The LaNiO3-δ electrode suffers from the degradation around 1073 K and shows poor reproducibility. Therefore, the phase stability window of LaNiO3-δ needs to be determined. The measurement by high temperature X-ray diffraction is important to determine the phase relationship of the LaNiO3-δ. Reference [1] D. O. Bannikov et al., J. Solid State Chemistry, 179(2006) 2723. [2] R. M. Garcia et al., App. Catalyst B :Environmental, 33 (2001) 49. Figure 1

γ‐Brasses with Spontaneous Magnetization: Atom Site Preferences and Magnetism in the Fe‐Zn and Fe‐Pd‐Zn Phase Spaces

AbstractThe γ‐brasses in the binary Fe‐Zn system were re‐examined as part of the discovery of a new series of Fe‐Pd‐Zn γ‐brass compounds in the Zn‐rich region of the Fe‐Pd‐Zn phase space. Fe2+xZn11–x, x = 1.60(4)–1.99(9), and FexPd2.2–yZn10.8–x+y, x = 0.08(5)–2.46(8); y = 0.00(1)–1.44(4), all crystallize in the cubic crystal system, space group I$\bar{4}$3m, with corresponding ranges of the lattice parameter of 8.977(1)–8.979(1) Å and 9.1028(4)–9.0347(2) Å as x increases, as determined by single‐crystal X‐ray diffraction. In the binary Fe‐Zn system, iron atoms preferentially occupy the outer tetrahedron (OT) sites and partially mix with zinc atoms at the inner tetrahedron (IT) sites in the 26‐atom γ‐brass clusters. At the refined upper limit of Fe content, Fe atoms also replace Zn atoms at the octahedron (OH) sites. For the ternaries FexPd2.2–yZn10.8–x+y, refined site occupancies revealed three distinct patterns: (i) at low Fe content (x &lt; 0.2), Fe/Pd and Pd/Zn mixing occurs, respectively, at the OT and OH sites; (ii) at intermediate Fe content (x ≈ 0.7), Fe/Pd and Fe/Zn mixing occurs, respectively, at the OT and IT sites; and (iii) at high Fe content (x &gt; 0.9), Fe mixes at the IT (with Zn), OT (with Pd), and OH (with Zn) sites. Magnetization measurements of the Fe‐rich (x &gt; 0.9) ternary phases indicate ferromagnetic or ferrimagnetic responses, which are consistent with the results of electronic structure calculations on “FePdZn11” used to model this phase. In addition, a Mulliken population analysis of the different sites in the 26‐atom γ‐brass cluster also provide a rationale for the complex behavior of atomic site occupancies vs. composition in both Fe2+xZn11–x and FexPd2.2–yZn10.8–x+y.

New Co–Pd–Zn γ-Brasses with Dilute Ferrimagnetism and Co2Zn11 Revisited: Establishing the Synergism between Theory and Experiment

A synergism between electronic structure theory and the targeted synthesis of new ternary γ-brass compounds is demonstrated in the Co–Zn system. Co2Zn11, which adopts a cubic γ-brass structure, is shown to be at the Zn-rich end of a homogeneity range that varies from 15.4 to 22.1 atom % Co. Four samples were examined by single-crystal diffraction, all of which crystallize in space group I4̅3m with the lattice parameter ranging from 8.9851(1) to 8.8809(1) Å as the Co content increases. In the 26-atom γ-brass clusters, Co atoms preferentially occupy the outer tetrahedron (OT) sites and then replace Zn atoms at the octahedron (OH) sites at higher Co concentrations. In addition, a small fraction of vacancies occurs on the inner tetrahedron (IT) sites. The electronic structure of Co2Zn11 shows two distinct pseudogaps near the Fermi level: one at 292 valence electrons per primitive unit cell and the other at 302–304 valence electrons per primitive unit cell. Using molecular orbital arguments applied to the body-centered cubic packing of the 26-atom Co4Zn22 γ-brass cluster, these pseudogaps arise from (i) splitting among the valence s and p orbitals, which gives rise to the Hume–Rothery electron counting rule, and (ii) splitting within the manifold of Co 3d orbitals via Co–Zn orbital interactions. Co2Zn11 is Pauli paramagnetic, although the density of states at the Fermi level is large, whereas Curie–Weiss behavior emerges for higher Co concentrations. Because Pd has a size and an electronegativity similar to those of Zn, and inspired by the pseudogaps in the electronic density of states curve of Co2Zn11, Pd-doped γ-brass compounds were designed and two new γ-brass compounds were obtained: Co0.92(2)Pd1.08Zn11 and Co2.50(1)Pd2.50Zn8. In these, the site preferences for Co and Pd can be rationalized by electronic structure calculations. The densities of states indicate that Co 3d states are the major contributors near their Fermi levels, with the Pd 4d band lying ∼2–3 eV below this. The magnetic properties of the Co–Pd–Zn γ-brasses are quite different from those of Co2Zn11: a giant magnetic moment on the Co atom is induced by the Pd atom, and Co2.50(1)Pd2.50Zn8 shows magnetization consistent with a dilute ferrimagnet. The results of first-principles calculations on two different models of the 26-atom γ-brass clusters indicate that intracluster Co–Co exchange is ferromagnetic, whereas intercluster Co–Co exchange is antiferromagnetic. These different magnetic exchange interactions provide rationalization for the high-temperature magnetization behavior of Co2.50(1)Pd2.50Zn8.

Study on the trap site in Ni–Cr alloy by first principle approach

First-principle approach for nickel chromium binary alloy with nitrogen was proposed to determine the trapping position of nitrogen in austenitic alloys. The energy of the alloy matrix with the nitrogen atoms at different positions relative to chromium atoms were calculated by density function theory. The calculated results show that the nitrogen atom diffusion along pathways through tetrahedron site consumed less energy than direct diffusion. Among the pathways through tetrahedron site, nitrogen atoms in octahedron sites without nearest neighboring chromium atom need least energy, while those with two chromium atoms need the most energy. Therefore, the trapping site for the nitrogen atom in the Cr containing alloy is the octahedron site with nearest neighboring two Cr atoms.

First-principles study of hydrogen storage and diffusion in B2 FeTi alloy

The structural, elastic properties and electronic structure of FeTi and its hydrides, and hydrogen diffusion behavior in FeTi alloy were investigated by the first-principles calculations within the frame work of density functional theory (DFT). The calculated lattice constants for FeTi were in good agreement with the experimental values, and the lattice distortion of hydrides increases with the rise of hydrogen content. The calculation results of formation enthalpy, fusion heat of hydrogen and octahedron interstice size showed that hydrogen atom is more likely to exist in the central site of octahedron with the frame of four Ti atoms and two Fe atoms. The independent elastic constants as well as polycrystalline elastic parameters (bulk modulus B, shear modulus G, Young’s modulus E, Poisson’s ratio ν and anisotropy value A) were calculated, and then the ductility and elastic anisotropy of FeTi and its hydrides were discussed in details. In addition, the density of states, electron density difference and population analysis for FeTi and its hydrides were also calculated, which reveals the underlying mechanism of structural stability and chemical bonding. Finally, the activation energy of next nearest neighbor diffusion for H between octahedron interstices in FeTi alloy was calculated to be 2.92eV, clarifying the atomic mechanism of hydrogen diffusion.

Effects of oxygen vacancy on impurity distribution and exchange interaction in Co-doped TiO2

Dilute magnetic semiconductors have auracted much attention because of their potential applications in spintronics. In this paper, the effects of oxygen vacancy on total energy and magnetism in Co-doped TiO2 are investigated using the density functional theory. The energy of the system with a shorter Co-Co distance is higher than that with larger Co-Co distance after introducing oxygen vacancy. Oxygen vacancy trends to congregate around the Co cations. Moreover, the strength of exchange couple reduces in the system with a shorter Co-Co distance after introducing oxygen vacancy. For the system with a larger Co-Co distance, the exchange couple between two Co impurities is anti-ferromagnetic if oxygen vacancy is located at the basal site of octahedron containing Co, and ferromagnetic if oxygen vacancy is located at the apical site of octahedron containing Co.

A New Three‐Dimensional Superstructure in Bafertisite

Abstract:A new superstructure was found in bafertisite [(Ba0.98Na0.02)1.00(Fe1.71Mn0.26Mg0.01)1.98 TiO [(Si1.82Ti0.04Al0.03Cr0.01)1.90O7](OH1.40F0.53Cl0.03)1.96] from Donghai County, Jiangsu Province, China. The occurrence of the superstructure reflections were observed by single crystal diffraction using a SMAR APEX CCD. The a*, b* and c* axis directions revealed extra weak reflection spots of the superstructure. The apparent 2a, 2b and 2c superstructure is monoclinic with unit cell a= 10.6502(15) Å, b= 13.7233(19)Å, c= 21.6897(3)Å, α= 90°, β= 94.698(3)°, γ=90°, space group Cm, Z= 16. If c* extra weak reflections are ignored, the secondary supercell gave a cell a= 10.6548(15)Å, b=13.7284(19)Å, c= 11.6900(17)Å, α= 90°, β= 112.322(28)°, γ= 90°, space group Cm, Z= 8. The basic subcell was obtained by ignoring all extra weak reflection spots and gave: a= 5.3249(17)Å, b=6.8669(22)Å, c= 10.8709(36) Å, α= 90°, β= 94.740(62)°, γ= 90°, space P21/m, Z= 2. The superstructure has been refined to R= 0.063 for 7805 [R(int)= 0.0266] unique reflections I&gt;2δ(I). The structure consists of an octahedra (O) sheet sandwiched between two heteropolyhedral (H) sheets. These sheets consist of Ti‐octahedra and twin tetrahedral disilicate groups [Si2O7]. The O sheet comprises (Fe,Mg)O4 octahedra. The large Ba cation is located in the interlayer area. The refined structure shows Fe, Mg are partly ordered. The shifting of the TiO6 octahedron and SiO4 tetrahedron sites in the sheet may be a consequence of the superstructure.