Tetrahedral Interstitial Sites Research Articles

First-principles insight of hydrogen dissolution and diffusion properties in γ-Al2O3

First-principles theory is applied to identify the structure of γ-Al2O3 first. Then, hydrogen dissolution and diffusion behavior was further studied to reveal the reason that permeation reduction factor (PRF) of γ-Al2O3 is lower than that of α-Al2O3. Our calculations show that the formation energy for Al vacancy at the octahedral interstitial site (VAl−OIS−3) is lower than that at the tetrahedral interstitial site (VAl−TIS−3), and are far lower than those of other defects in both defective spinel and non-spinel γ-Al2O3. Thus, the stability of VAl−OIS−3 is higher than that of VAl−TIS−3 in both Al-rich and O-rich environments. However, when hydrogen atoms are introduced into γ-Al2O3, the stability of [VAl-TIS-H]−2 is higher than that of [VAl-OIS-H]−2 in the defective spinel γ-Al2O3 structure, while the opposite is true for the non-spinel γ-Al2O3 structure. The vibrational frequencies for OH− in [VAl-TIS-H]−2 and [VAl-OIS-H]−2 for defective γ-Al2O3 are calculated to be 3608 cm−1 and 3374 cm−1, respectively, which is in excellent agreement with the infrared (IR) absorption peaks observed at ∼3500 and ∼3300 cm−1. The defective spinel model is more suitable for describing the γ-Al2O3 structure. In defective spinel γ-Al2O3, there are many native Al vacancies, which are arranged in a straight line along the [21¯1¯0] direction. The migration barrier for H diffusion along these native Al vacancies is so low that these Al vacancies can provide a rapid diffusion channel for H. Thus, the permeability of H in γ-Al2O3 is much higher than that in α-Al2O3 leading to a lower permeation reduction factor (PRF). Our results can provide not only a sound theoretical explanation for the low PRF of γ-Al2O3 but also a direction to improve the efficiency in preventing H permeation through FeAl/Al2O3 tritium permeation barriers (TPBs).

Hydrogen at symmetric tilt grain boundaries in aluminum: segregation energies and structural features

Aluminum is envisioned to be an important material in future hydrogen-based energy systems. Here we report an ab initio investigation on the interactions between H-atoms and common grain boundaries (GBs) of fcc Al: Σ9, Σ5, Σ11 and Σ3. We found that upon segregation to the GBs, single H-atoms can cause displacement of Al-atoms. Increasing their concentration revealed large cooperative effects between H-atoms that favor the segregation when other H-atoms are bound at neighboring sites. This makes these GBs able to accommodate high concentrations of H-atoms with considerable segregation energies per atom. Structural analyses derived from Laguerre–Voronoi tessellations show that these GBs have many interstitial sites with higher symmetry than the bulk tetrahedral interstitial site. Many of those sites have also large volumes and higher coordination numbers than the bulk sites. These factors are the increased driving force for H-atom segregation at the studied GBs in Al when compared to other metals. These GBs can accommodate a higher concentration of H-atoms which indicates a likely uniform distribution of H-atoms at GBs in the real material. This suggests that attempting to mitigate hydrogen uptake solely by controlling the occurrence of certain GBs may not be the most efficient strategy for Al.

Computational modeling of the defect structure, hyperfine and magnetic properties of the Mn2+-doped magnetite of the composition MnxFe3-yO4 (y = ⅔ x)

The defect structure, hyperfine and magnetic properties of Mn2+-doped Fe3O4 of the composition MnxFe3-yO4(y=⅔x) are modeled using atomistic and DFT calculations. The atomistic simulations show the substitution of the Mn2+ ions for Fe3+ ones at the tetrahedral sites to be energetically favorable than their substitution at the octahedral sites. These Mn2+ impurities are charge-balanced by the occupation of either Mn2+ or Fe 3+ ions of interstitial tetrahedral sites. The method of GGA with on-site Coulomb interaction approximation for the exchange-correlation potential is used to calculate the electronic structure, hyperfine and magnetic moments of the structurally most preferred models. The results obtained show the model in which all the Mn2+ ions substitute for tetrahedral Fe 3+ ions with Fe3+ ions expelled to interstitial tetrahedral sites to be consistent with the observed experimental trends of the hyperfine and magnetic properties.

The bonding of H in Zr under strain

Accurate computer simulation is important for understanding the role of irradiation-induced defects in zirconium alloys found in nuclear reactors. Of particular interest is the distribution and trapping of hydrogen, and the formation of zirconium hydride. These simulations require an accurate representation of Zr-H bonding in order to predict the behaviour of H around atomic-scale defects, dislocation lines, and dislocation loops. Here we explore the bonding of H in Zr under strain, how well it is represented by state-of-the-art Embedded Atom Method (EAM) potentials, and what physics is needed for an accurate representation in a Linear Combination of Atomic Orbitals (LCAO) Density Functional Theory (DFT) framework. For H in dilute solution under hydrostatic strain in the range -10% to +10%, solution energies and Zr-H bond lengths computed using EAM potentials are found to be in poor agreement with plane-wave DFT results. We note that the bond lengths are in a poor agreement even in equilibrium. LCAO basis sets are used to explore the importance of electron distribution around H atoms, and the transfer of electrons between H and Zr. The electron distribution around H atoms is found to be important to the explanation of the difference between octahedral and tetrahedral interstitial sites for H, with H in a tetrahedral site having very similar bonding to H in zirconium hydrides. The interatomic electron transfer has a smaller impact but is needed for maximum accuracy.

Effects of iron impurities on the photoelectric response characteristics of silicon.

Since Fe contamination is easily mixed into Si-based photoelectric materials during the fabrication process, the relative changes of energy levels and photoelectric properties of Si material mixed with Fe impurities in different occupancy sites are studied. Based on the first principle and photoelectric response theory, an analysis model of the response characteristics of Si material mixed with Fe impurities is established. The changes of the material's electronic structure are calculated, and the effects of different occupancy sites of Fe impurities on the photoelectric response characteristics of materials are comparatively analyzed. Results show that when the Fe atom occupies the tetrahedral interstitial site in Si, the energy band structure and response characteristics of the material are relatively obviously affected. In this case, the impurity energy band introduced by the Fe impurity passes through the Fermi level, leading to the disappearance of the band gap. The absorption of Si material outside the response band is significantly enhanced. And a new absorption peak is generated at about 1530nm, with an absorption coefficient of about 25513 cm-1. Thus, the Si material can produce a relatively strong response to the light beam outside the response band. In the meantime, the saturation threshold of the Si-based photoreceptor is significantly lower than that of the other two position types. For the irradiation light at the wavelength of 1319nm, the saturation power is only 0.0035 W∙cm-2. The analysis results provide a reference for the application and development of photoelectric devices.

Abnormal trapping of hydrogen in the elastic stress field of dislocations in body-centered cubic iron

It has been widely accepted that the tetrahedral interstitial site is the normal lattice trapping site for H atoms in the body-centered cubic iron. In this work, we found H trapped in a tetrahedral site is unstable in the dislocation elastic stress field. By contrast, the octahedral site is energetically preferred. This abnormal trapping phenomenon was rooted in the H-dislocation elastic interaction and zero-point quantum vibration of the H atom. At the abnormal trapping state, the interaction between H-edge dislocation and H-screw dislocation was identified to be at a similar level. As a result, the diffusivity of H around dislocation is enhanced by the abnormal trapping, and the local H content is much higher than the prediction with the assumption of normal trapping. Our results suggest that the dislocation motion can shuffle the trapping position of H atoms and assist the immature failure in the hydrogen environment.

Effects of surface hydroxyl ions on the color of sintered Al2O3 characterized by X-ray photoelectron and infrared spectroscopy

Motivated by the different chromaticity present in the sintered Al2O3 prepared from various same-grade powders, this study examined the probable factor that affects the color by surface chemistry. Only the surface of the sintered Al2O3 changed to a darker ivory color upon ultraviolet (UV) irradiation, which recovered its initial color after a couple of weeks, implying possible surface interaction. Deconvoluting the O 1s XPS peak into the O-1 and O-2 components showed that the amount of surface hydroxyl ions varies according to the powder type and decreases by UV irradiation. In addition, hydroxyl ions adsorbed on the surface of sintered Al2O3 were distinguished by Fourier transform infrared (FT-IR) spectroscopy. Based on the correlation between the XPS O-2 area and the intensity of each FT-IR peak, the hydroxyl ions attached to Al3+ in the tetrahedral interstitial sites most likely affect the color of sintered Al2O3.

Solubility and mechanical properties of hydrogen / carbon in Mo–Ta alloy

The solubility, diffusivity, electronic and mechanical properties of Hydrogen (H) and Carbon(C) impurity atoms in Mo–Ta alloy were studied by first-principles calculation. The solubility energy results of C/H in Mo–Ta alloy show that H tends to form in tetrahedral interstitial site (TIS), in contrast to C which tends to octahedral interstitial site (OIS). The diffusion barrier of H/C atoms in Mo–Ta alloy was analyzed, and the results showed that Ta atom hinder the diffusion of H atom, but promote the diffusion of C atom. The electronic structure of Mo–Ta alloys was analyzed by the projected density of states (PDOS). On the other hand, the addition of C/H atoms will reduce the strength and improve the ductility of Mo–Ta alloys. For the interstitial atoms, the effect of C on the alloy is stronger than that of H due to the strong electronic interaction between C and Mo–Ta alloy. These results will help us understand the basic behavior of Mo–Ta alloy and the research of the first wall material of fusion reactor.

First-principle study on the effects of hydrogen in combination with alloy solutes on local mechanical properties of steels

The first-principle calculation method has been utilized to study the effects of hydrogen in combination with alloy elements on the mechanical properties of iron-based super-cells, with the aim to bring new insight in the design of hydrogen-resistant steels. It is found that hydrogen prefers the second and third tetrahedral interstitial sites of the alloy solutes. Nb/Zr/Mo/W/Al alloyed super-cells dropped the most in the bulk modulus by hydrogen addition. Ni/Nb/Cu/Co/Zr dropped the most in the shear modulus/Young's modulus/hardness. The underlying influencing mechanisms on the elastic mechanical properties are discussed in perspective of atomic parameters/density of states/charge density/difference charge density/Bader charge analysis.

Oxygen point defect stabilized metastable M3‐phase VO2 films

Point defects can stabilize various metastable polymorph phases of complex oxides showing unique physical properties, even though their role in the stabilization of the metastable phase is unclear. In this study, we examined the role of oxygen point defects in stabilizing a polymorph M3-VO2 phase and its transition characteristics. Raman spectroscopy revealed the M3-VO2 phase at the ambient environment for the VO2 film grown on c-Al2O3 substrate at a higher oxygen flow rate. High-resolution XRD and DFT calculation exhibited that the M3-VO2 phase was stabilized by oxygen point defect incorporated into the tetrahedral interstitial site, the most stable defect type in the monoclinic VO2 system. The M3-phase had a higher IMT temperature and a sharper IMT than the M1-phase due to the decrease in interdimer hopping energy and the small energy barrier for the IMT by the slightly zigzag V-V chain. Furthermore, the temperature-dependent Raman spectra showed the unprecedented structural phase transition from the M3-VO2 phase to the R-phase without an intermediate state. Overall, these findings provide fundamental information on the role of oxygen point defects in realizing metastable polymorph phases in promising new technological materials.

First-principles calculations of transition elements interaction with hydrogen in vanadium

We performed systematic first-principles calculations to investigate the interactions of 26 transition elements (TEs) with hydrogen (H) and vacancy in vanadium (V). In the presence of 26 substitutional solute elements (Sc,Ti,Mn,Cr,Co,Fe,Cu,Ni,Y,Zr,Mo,Nb,Rh,Ru,Pd,Tc,Ag,La,Hf,W,Ta,Os,Re,Pt,Ir,Au), H still prefers to occupy the tetrahedral interstitial sites. H is unstable for solute elements at the nearest octahedral interstitial sites except for Ni and Cu. Then we calculated the binding energies of TEs and H with their distances. H atoms are attractive with early three 3d/4d/5d TEs (Sc/Ti/Y/Zr/La/Hf) and repulsive with other TEs. And we predicted the effect of solid solution elements on H diffusivity, Sc/Y/Zr/La/Hf can significantly reduce the effective diffusivity of H, while other elements have little effect on it. In addition, most of TEs can stabilize TE-vacancy-H clusters, the interactions of all TE-vacancy-H clusters are attractive and decrease with the increase of atomic number in the same period. In the case of pre-existing TE-vacancy complex defects, most of TEs can reduce the binding strength of vacancy to H. These results deepened the understanding of the TEs-H interactions and H retention in V alloys under irradiation.

Revealing effects of common nonmetallic impurities on the stability and strength of Cu–Sn solder joints: A first-principles investigation

Nonmetallic elements can affect the stability and usability of Cu–Sn solder joints intensively. The electronic structure, adhesion work, and tensile strength of Cu(111)/Cu3Sn(001) interface doping with H, C, O and S have been systematically investigated herein using the first-principles calculations. Cu(Cu)-terminated interface is the most stable among the clean Cu(111)/Cu3Sn(001) interfaces. In the non-metallic atoms doping Cu(111) surface system, H atom tends to occupy tetrahedral interstitial sites, while C, O and S atoms prefer to octahedral interstitial sites. The calculated segregation heat (ΔGseg) indicates that H and O atoms tend to stay in the interior, while C and S atoms tend to segregate to the Cu(111)/Cu3Sn(001) interface. From adhesion works, the order of binding strength is SOct > clean interface > OOct > COct > HTet. Finally, electronic structures of doping systems are analyzed to reveal the effects of nonmetallic impurities on the stability and strength of Cu–Sn solder joints. These results will provide guidance for the design of modified Cu–Sn solder joints.

SYNTHESIS AND STRUCTURAL ANALYSIS OF NICKEL FERRITE SYNTHESIZED BY CO-PRECIPITATION METHOD

The Nickel ferrite has been synthesized by co-precipitation method. X-ray diffraction pattern confirms the formation of cubic spinel structure with lattice constant 8.347Å. Structural properties like X-ray density, average crystalline size, bond length, dislocation density and microstrain have been studied. The scanning electron microscope images show grain of bead structures. The Fourier transform infrared spectroscopy spectrum of nickel ferrite under investigation reveals the formation of a cubic spinel structure showing two significant absorption bands, corresponding to high frequency band ν1and low frequency band ν2arising from tetrahedral (A) and octahedral (B) interstitial sites respectively

Theoretical investigations of hydrogen absorption in the A15 intermetallics Ti3Sb and Ti3Ir

Abstract Ti3Sb and Ti3Ir adopt the A15 (Cr3Si type) structure and are reported to incorporate hydrogen atoms to an extent, respectively, of Ti3SbH∼3 and Ti3IrH3.8. First-principles electronic structure calculations were performed to identify factors contributing to the difference in maximum hydrogen composition for these two intermetallic compounds. Relative energies and changes in energy densities of states and crystal orbital Hamilton populations upon H insertion in the intermetallic compounds were examined. In both compounds, hydrogen atoms are attracted to [Ti4] tetrahedral interstitial sites over any others. The natures of metal-hydrogen and metalloid-hydrogen bonding and the effects of hydrogen insertion on metal-metal and metal-metalloid bonding have an influence on the maximum hydrogen contents for Ti3Sb and Ti3Ir.

Magnetic transitions with magnetocaloric effects near room temperature related to structural transitions in Y0.9Pr0.1Fe2D3.5 deuteride

The structural and magnetic properties of Y0.9Pr0.1Fe2D3.5 deuteride have been investigated by synchrotron and neutron diffraction, magnetic measurements, and differential scanning calorimetry. Deuterium insertion induces a 23.5% cell volume increase and a lowering of crystal symmetry compared to the cubic C15 Y0.9Pr0.1Fe2 parent compound (Fd-3m SG). The deuteride is monoclinic (P21/c SG) below 330 K and undergoes a first-order transition between 330 and 350 K toward a pseudo-cubic structure (R-3 m SG) with TO–D = 342(2) K. In both structures, the D atoms are located in 96% R2Fe2 and 4% RFe3 tetrahedral interstitial sites (R = Y0.9Pr0.1). The compound is ferromagnetic, accompanied by a magnetostrictive effect below TC = 274 K. The analysis of the critical exponents indicates a second-order type transition with a deviation from the isotropic 3D Heisenberg model toward the 3D XY model. This implies an easy plane of magnetization in agreement with cell parameter variation showing planar magnetic orientation. A weak magnetic peak is even observed at the order–disorder transition with a maximum at 343 K. Magnetic entropy variations are characteristic of direct and inverse magnetocaloric effects at TC and TO–D, respectively.

Structure–property relationship and spectroscopic studies of BaO–B2O3 oxide glasses containing ZnO for optical applications

In the present work, samples of barium borate glasses containing different molar ratios of ZnO were prepared using the conventional melt-quenching technique. X-ray diffraction studies confirmed the amorphous nature of the studied materials. Structural analysis using FT-Infrared confirmed the incorporation of the Zn atom in the glass matrix as a ZnO4 unit and the existence of ZnO in the tetrahedral interstitial sites. TEM images for the studied materials show that the average particle size is about 20nm and the ZnO4 structural units reside in the glass matrix with high homogeneity. Density and molar volume studies manifested an increase in the density as the amount of ZnO increases and a subsequent decrease in the molar volume. UV absorption studies showed a blue shift in the absorption peak from 343nm to 315nm. Additionally, an observed reversible behavior around 338nm divides the absorption range into high energy and low energy regions. In the high energy region, both absorbance, absorption coefficient, and refractive index increased with ZnO content increment. In the low energy region, this behavior was reversed, and the calculated values of the refractive index agreed with the measured ones in this region. Studies of optical dielectric parameters showed a Debye-type relaxation process in which increasing the ZnO content shortens the sample relaxation time. Our results, collectively, suggest the studied materials for several optoelectronic device applications such as high-energy optical filters, optical switches, and frequency converters.

First‐Principles Investigation of the Effect of Vanadium Doping on Hydrogen Incorporation in Tungsten

AbstractUsing the first‐principles method based on the density functional theory, hydrogen diffusion, electron structure, and mechanical properties of hydrogen impurity in W15V structure are investigated in this paper. It is found that in W‐V lattice the single H atom tends to be located in the tetrahedral interstitial site nearest to V atom with the solution energy of 0.614 eV. The calculation of energy barriers shows that the V doping can be helpful to trap H atoms near the V atom. The analysis of the elastic moduli shows that the V‐alloying can decrease the strength and improve the ductility of pure W. In addition, the H impurity further enhances this situation. Besides, the effect of H impurity on pure W is greater than W15V structure. The results of this study offer a useful database for the research of W‐based alloys for plasma facing materials (PFMs).

Impact of cobalt substitution on cation distribution and elastic properties of Ni–Zn ferrite investigated by X-ray diffraction, infrared spectroscopy, and Mössbauer spectral analysis

The present study deals with site occupancy of cobalt in Ni–Zn ferrite investigated by in-field Mössbauer spectroscopy at 300 K, 77 K, and 5 K coupled with X-ray diffraction, Fourier transform infrared spectroscopy, and Rietveld analysis. The cation distribution for the whole series of compositions was proposed by consideration of the amounts of iron ions present at tetrahedral and octahedral interstitial sites. Confirmation of the proposed distributions was achieved by quantitative estimation of the lattice constants for these compositions and comparison with experimental values. Pauling electronegativities remain almost constant, indicating no significant variation in the strength of the ionic bond with cobalt substitution. The observed decrease in the Debye temperature calculated with both the Waldron formula and the Anderson formula suggests that as a result of cobalt doping, there is slight enhancement in lattice vibrations, which is attributed to the minor weakening of the interatomic bonding between the atoms in the spinel lattice.

Missing Piece in the Crystal Chemistry of Zn-Sb Secondary Phases in ZnO-Sb2O3-Bi2O3 Varistor Ceramics: Orthorhombic β-Zn7Sb2O12. An Experimental and Theoretical Study of the Crystal Structure and Its Thermal and Vibrational Spectroscopic Characterization.

The ternary Zn-Sb oxide β-Zn7Sb2O12 was prepared by solid state synthesis from ZnO and Sb2O3 at 1250 °C and a reaction time of 168 h. The crystal structure of β-Zn7Sb2O12 was solved from powder diffraction data by direct-space methods and was refined by the Rietveld technique. The title compound β-Zn7Sb2O12 crystallizes in the orthorhombic space group Cmme with a = 1210.6(1) pm, b = 1856.7(1) pm, c = 852.2(1) pm, V = 1.9154(1) nm3, and eight formula units per unit cell. The structure can be described as a distorted cubic closest packing of oxide ions with Zn2+ and Sb5+ in the tetrahedral and octahedral interstitial sites. According to group-subgroup relations, the anion packing is directly derived from the spinel structure of α-Zn7Sb2O12; however, the occupation pattern of the interstitials is completely different than in the spinel structure type. The most prominent structural feature in β-Zn7Sb2O12 is the clustering of all [ZnO4] polyhedra into a [Zn7O20] moiety representing an excerpt from the sphalerite structure. The structural model is corroborated by vibrational spectroscopy as well as density functional theory calculations. Thermal analysis reveals an irreversible phase transition of β-Zn7Sb2O12 into the α-polymorph at 1330 °C.

First-Principles Study of the Effects of Carbon, Nitrogen, and Oxygen on Helium Behavior in Body-Centered-Cubic Vanadium

Density functional theory calculations were used to study the effects of inherent impurities C, N, and O on the stability and the self-trapping of interstitial He atoms in body-centered-cubic vanadium (V). The most stable site for the He atom nearby C, N, and O is the tetrahedral interstitial site (T-site) rather than the octahedral interstitial site (O-site). The presence of C, N, or O impurities reduces the stability of He in the T-site according to the calculated formation energies. The addition of C and O atoms is beneficial for He self-trapping while the addition of the N atom prevents He self-trapping in vanadium. The stable configurations for Xn-vacancy1 (XnVa1) are C2Va1, N2Va1, and O2Va1. The trapping energies of multiple He atoms captured by XnVa1 are investigated. Our results show that the presence of C, N, and O reduces vacancy trapping of He atoms. Our findings provide further understanding on the behavior of He atoms in vanadium with the influence of C, N, and O.