Tetrahedral Vacancies Research Articles

Surface structure and catalytic properties of MoO3/CeO2 and CuO/MoO3/CeO2

XRD, LRS, TPR and in situ NH3 adsorption FT-IR were used to investigate the dispersion state of the copper oxide and molybdena species of MoO3/CeO2 and CuO/MoO3/CeO2 catalysts as well as their surface acidity. The results showed that the molybdena monolayer modification promoted the dispersion of CuO due to the formation of new tetrahedral vacancies. Meanwhile, CuO changed the structure of molybdenum species and then influenced the surface acidity of the samples. A detail discussion about the possible model of the surface structure of the catalyst was presented. In addition, combining with the in situ NH3 adsorption FT-IR, the relationships between the activities for ‘‘NO+NH3+O2’’ reaction and surface acid properties (Brønsted and Lewis acid sites) of the catalysts were discussed.

Effect of metal–support interaction on the catalytic performance of Ni/Al 2O 3 for selective hydrogenation of isoprene

The effect of the interaction between metal and support on the performance of different phases alumina supported nickel catalysts was demonstrated in liquid phase selective hydrogenation of isoprene in simulated gasoline. Catalysts supported on γ-Al 2O 3 and κ-Al 2O 3 were characterized by BET, XRD, TPR, and XPS techniques. The results showed that a “surface spinel”, NiAl 2O 4 was formed. The supported nickel ions preferentially incorporate into the tetrahedral vacancies of γ-Al 2O 3 support to form SMSI (strong metal–support interaction), while WMSI (weak metal–support interaction) was formed on κ-Al 2O 3 support with little tetrahedral vacancy. When there were enough hydrogenation sites, the Ni/γ-Al 2O 3 catalysts with SMSI which can resist carbon deposition, performed higher isoprene conversion, higher stability and lower selectivity than the same nickel loading Ni/κ-Al 2O 3 samples with WMSI. The weak interaction had a positive effect on the formation of coke, which was mainly related to the hydrogenolytic sites, leading to main reaction with high mono-olefins selectivity with isoprene conversion decrease evidently.

Cation vacancy order in the K0.8+xFe1.6−ySe2 system: Five-fold cell expansion accommodates 20% tetrahedral vacancies

Ordering of the tetrahedral site vacancies in two crystals of refined compositions K0.93(1)Fe1.52(1)Se2 and K0.862(3)Fe1.563(4)Se2 produces a fivefold expansion of the parent ThCr2Si2 unit cell in the ab plane which can accommodate 20% vacancies on a single site within the square FeSe layer. The iron charge state is maintained close to +2 by coupling of the level of alkali metal and iron vacancies, producing a potential doping mechanism which can operate at both average and local structure levels.

Quantum instanton evaluations of surface diffusion, interior migration, and surface-subsurface transport for H/Ni

The quantum instanton approximation is extended to investigate dynamical processes of hydrogen on surface, from surface to subsurface, and between interior sites in nickel lattice. The path integral Monte Carlo and adaptive umbrella sampling techniques are employed to manipulate the quantum instanton formula. The free energy profiles along reaction paths, temperature dependence of free energies, and rates as well as diffusion coefficients are calculated for each process. The results manifest that the motions of nickel atoms beneath the surface have little effect on the hydrogen diffusion on Ni(111), and the hydrogen at the fcc binding site is much easier to get into bulk nickel than the one at the hcp site. The temperature dependence of free energy profiles also reveals that the hydrogen in the subsurface octahedral vacancy and interior tetrahedral vacancy becomes unstable at low temperatures, which proposes a temperature dependence of reaction mechanism. In addition, the relaxations of the lattices dramatically lower the free energy barriers except for the process of the hydrogen diffusion on Ni(111). The quantum motions of the lattice atoms affect the free energies little at 300 K, but they hinder the rates by 20%-40% compared with the classical motions of lattice atoms.

Analysis of Ga coordination environment in novel spinel zinc gallium oxy-nitride photocatalysts

Novel zinc gallium oxy-nitrides, with the spinel structure and visible light band gaps, have been synthesized by nitridation of zinc gallate produced by sol–gel synthesis. These spinel oxy-nitrides have band gaps of 2.5 to 2.7 eV, surface areas of 16 to 36 m2 g−1, and nitrogen content less than 1.5%. They degrade methylene blue dye in visible light. The reduction in band gap is associated with the incorporation of nitrogen in the zinc gallate structure and corresponding changes in the anion position parameter as well as the presence of a small fraction of gallium tetrahedral centers and anion vacancies. While spinel oxy-nitrides are produced under nitridation at 550 °C, at higher temperatures they are consumed to form wurzitic oxy-nitrides. The wurtzite materials also have band gaps less than 3 eV but their surface areas are 2 to 5 m2 g−1. The changes associated with the gallium coordination as the spinel zinc gallate precursor transforms into the spinel oxy-nitride at 550 °C, and further changes into the wurtzite oxy-nitride at 850 °C are studied through X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, neutron powder diffraction, X-ray absorption spectroscopy and other techniques. The protocol developed in this report opens an avenue for the synthesis of semiconductors having the spinel crystal structure and band gaps engineered to the visible region with potential applications for opto-electronic devices and photocatalytic processes.

Synthesis of nanocrystalline Cd–Zn ferrite by ball milling and its stability at elevated temperatures

High energy ball milling of stoichiometric (0.5:0.5:1 mole fraction) mixture of CdO, ZnO and α-Fe 2O 3 powders in air at room temperature results in formation of a non-stoichiometric Zn-rich (Zn,Cd)Fe 2O 4 phase with normal spinel structure having tetrahedral vacancies. The ferrite phase is initiated at 1 h of milling and after 25 h milling, 0.96 mole fraction of ferrite is formed and 0.04 mole fraction of CdO phase remained unreacted. The phase stability study of nanocrystalline non-stoichiometric (Zn,Cd)Fe 2O 4 powder annealed at elevated temperatures reveals that the Zn-rich ferrite phase remained stable up to 973 K and then slowly transformed towards Cd-rich (Cd,Zn)Fe 2O 4 phase following the release of divalent cations from ferrite lattice of normal spinel structure. The non-stoichiometric ferrite phase with almost similar composition has also been obtained by conventional ceramic route by sintering the same stoichiometric mixture at 973 K for 1 h. Microstructure characterization in terms of several lattice imperfections, relative phase abundances, cation distribution, and phase stability studies of unmilled, ball-milled and annealed samples is made by employing the Rietveld's structure refinement methodology using X-ray powder diffraction data. The analysis reveals that the particle size of ferrite phase reduces to ∼7 nm after 25 h of milling and after annealing at 1273 K for 1 h it grows up to ∼700 nm. However, in case of ferrite prepared by ceramic route it grows up to ∼250 nm which is quite less than the annealed ball-milled samples.

Correlation between self-diffusion in Si and the migration mechanisms of vacancies and self-interstitials: An atomistic study

The migration of point defects in silicon and the corresponding atomic mobility are investigated by comprehensive classical molecular-dynamics simulations using the Stillinger-Weber potential and the Tersoff potential. In contrast to most of the previous studies both the point defect diffusivity and the self-diffusion coefficient per defect are calculated separately so that the diffusion-correlation factor can be determined. Simulations with both the Stillinger-Weber and the Tersoff potential show that vacancy migration is characterized by the transformation of the tetrahedral vacancy to the split vacancy and vice versa and the diffusion-correlation factor ${f}_{V}$ is about 0.5. This value was also derived by the statistical diffusion theory under the assumption of the same migration mechanism. The mechanisms of self-interstitial migration are more complex. The detailed study, including a visual analysis and investigations with the nudged elastic band method, reveals a variety of transformations between different self-interstitial configurations. Molecular-dynamics simulations using the Stillinger-Weber potential show that the self-interstitial migration is dominated by a dumbbell mechanism, whereas in the case of the Tersoff potential the interstitialcy mechanism prevails. The corresponding values of the correlation factor ${f}_{I}$ are different, namely, 0.59 and 0.69 for the dumbbell and the interstitialcy mechanisms, respectively. The latter value is nearly equal to that obtained by the statistical theory which assumes the interstitialcy mechanism. Recent analysis of experimental results demonstrated that in the framework of state-of-the-art diffusion and reaction models the best interpretation of point defect data can be given by assuming ${f}_{I}\ensuremath{\approx}0.6$. The comparison with the present atomistic study leads to the conclusion that the self-interstitial migration in Si should be governed by a dumbbell mechanism.

First principles site occupation and migration of hydrogen, helium, and oxygen in β-phase erbium hydride

First principles density functional methods were used to investigate the atomistic behavior of hydrogen, helium, and oxygen in β-phase ErH2. The ground state for hydrogen was indeed determined to be the tetrahedral position as commonly assumed, but if the surrounding tetrahedral sites are filled, any additional hydrogen will occupy the octahedral site. Only a small amount of thermally generated tetrahedral-vacancy octahedral-occupancy pairs are predicted at equilibrium since the formation energy is 1.21 eV. Other possible scenarios that result in octahedral hydrogen occupation include a H/Er ratio >2.0 and the presence of oxygen in the lattice. Our calculations indicate that oxygen impurities will reside in tetrahedral sites, even if that site is already occupied and hydrogen must be displaced into a neighboring octahedral site. Oxygen will migrate at moderate temperatures by jumping between tetrahedral and octahedral sites. The extent of hydrogen self-diffusion will depend on the concentration of tetrahedral vacancies and/or octahedral hydrogen and therefore can be modified by changing the H/Er ratio or by impurities (such as oxygen) that create octahedral hydrogen occupation. In samples where some of the hydrogen is replaced with tritium, helium generated by tritium decay will favor a tetrahedral site left vacant by a transmuted tritium. The barrier to helium migration between two unoccupied neighboring tetrahedral sites is 0.49 eV, where the path maximum corresponds to the octahedral site. If an extended network of neighboring vacancies exists, the relatively small barrier provides that helium may move throughout that network at room temperature. Given enough energy to escape the tetrahedral site(s), 1.31 eV, helium may continue to migrate by a 0.88 eV concerted-motion mechanism—temporarily displacing hydrogen as it moves between empty octahedral sites and filled tetrahedral sites.

Dehydroxylation of omphacite of eclogite from the Dabie-Sulu

Dehydroxylation of omphacites in eclogites from UHP Dabie-Sulu terrain has been investigated using a combined method of petrological observations, mineral chemistry, heat treatment, Mössbauer, and micro-FTIR spectroscopy. Progressive heating of omphacite in air up to 1100 °C results in complex, multi-stage processes involving oxidation and dehydrogenation at different temperatures. During heating, Fe 2+ at the M1 site is oxidized to Fe 3+, accompanied by a decrease in the intensity of the 3460 cm − 1 OH band. This reaction can be described as Fe 2+( M1) + (OH) − = Fe 3+( M1) + O 2−(O3) + 1/2H 2, and most likely represents loss of H bonded to O1 adjacent to the M1 site. At higher temperatures, continued oxidation results in further dehydroxylation due to H loss from OH bands 3530 cm − 1 and 3630 cm − 1 . These bands represent H defects with higher thermal stability/slower diffusivity. In this case, oxidation of Fe 2+ provides a means for charge-balancing H loss over the whole crystal structure, rather than locally. OH bands at 3530 and 3630 cm − 1 are consistent with H incorporation onto O2, with vibration of OH dipoles towards adjacent tetrahedral vacancies. Rapid loss of H during annealing at even low temperatures implies that significant H loss from eclogites occurs during exhumation. Furthermore, changes in relative OH band intensity are expected during exhumation due to different thermal behaviour of H defects in omphacite. As such, the ratio of OH bands in omphacite spectra could be used to deduce exhumation conditions.

Energetics of Aluminum Vacancies and Incorporation of Foreign Trivalent Ions in γ‐Al2O3: An Atomistic Simulation Study

Atomistic simulation methods based on pair‐wise interatomic potentials and energy minimization have been applied to elucidate the energetics of cation vacancies and the incorporation of 13 trivalent M3+ cations (Cr3+, Ga3+, Fe3+, Lu3+, Yb3+, Er3+, Y3+, Tb3+, Gd3+, Eu3+, Sm3+, Nd3+, La3+) in γ‐Al2O3. Calculations have been carried out using Al64O96 defect spinel supercells containing eight aluminum vacancies. The lowest energy configurations correspond to a random distribution of tetrahedral and octahedral vacancies. The energy gain in comparison with exclusive tetrahedral or octahedral vacancies is rather small (0.03 and 0.09 eV/Al2O3, respectively). Unit cell volume, density, and lattice properties of optimized structures are in good agreement with the experimental values or the results of high‐quality density functional theory calculations. The trends observed for the solution energy of the M2O3 oxides in the supercell with minimum energy indicate the preferential incorporation of the foreign ions at the tetrahedral site and an increase of the solubility of M2O3 in the defect spinel in comparison with α‐Al2O3. Configurations with the lowest energy have negative solution energies and, consequently, incorporation of trivalent ions can improve the thermodynamic stability of γ‐Al2O3 in comparison with α‐Al2O3 and increase the γ→α transition temperature.

Unified model of multichannel positron annihilation in nanoporous magnesium aluminate ceramics

The unified multichannel positron annihilation model in humidity-sensitive nanoporous MgAl2O4 ceramics with a spinel structure was generalized. It is shown that this model unifies the channels of positron trapping and ortho-positronium decaying. Within this approach, the first component in the lifetime spectra reflects microstructure specificity of the spinel structure with character octahedral and tetrahedral cation vacancies, the second component responsible to extended defects near intergranual boundaries and the third component correspond to "pick-off" annihilation of ortho-positronium in the water-filled nanopores of ceramics.

Influence of Nickel Salt Precursors on the Hydrogenation Activity of Ni/γ-Al2O3 Catalyst

Abstract The structure and catalytic properties of the supported Ni/γ-Al 2 O 3 catalyst prepared with different nickel salt precursors for hydrogenation of a-pinene have been studied using X-ray diffraction, UV–VIS diffuse reflectance spectroscopy, temperature-programmed reduction, CO chemisorption, and microreactor tests. It has been shown that the catalytic hydrogenation activity of the Ni/γ-Al 2 O 3 catalyst increases with increasing Ni loading, and the catalytic activity of the nickel acetate-derived Ni/γ-Al 2 O 3 catalyst is much higher than that of the nickel nitrate-derived catalyst. The catalyst characterization results indicate that the different catalytic hydrogenation activity of Ni/γ-Al 2 O 3 prepared with different nickel salt precursors is correlated to the different ratio of Ni 2+ ions in the tetrahedral and octahedral vacancies of γ-Al 2 O 3 and the different reduction degree of the NiO/γ-Al 2 O 3 precursors. When the nickel ion loading is far below the dispersion capacity, the supported nickel ions preferentially incorporate into the tetrahedral vacancies of γ-Al 2 O 3 . With increasing nickel loading, the ratio of Ni 2+ ions that incorporate into the octahedral vacancies of γ-Al 2 O 3 increases. The dispersion capacity of nickel ions on γ-Al 2 O 3 derived from nickel acetate is lower than that of the sample derived from nickel nitrate due to the greater shielding effect of acetate anions than nitrate anions. Moreover, the ratio of octahedral Ni 2+ to tetrahedral Ni 2+ on the nickel acetate-derived sample is higher than that on the nickel nitrate-derived sample because of the lower density of octahedral vacancies on γ-Al 2 O 3 . As octahedral Ni 2+ ions are easier to be reduced to the metallic state than tetrahedral Ni 2+ ions, the catalytic activity of the Ni/γ-Al 2 O 3 catalyst prepared with the nickel acetate precursor is much higher than that of the catalyst prepared with the nickel nitrate precursor with the same nickel loading.

Synthesis, Crystal Structure and Magnetic Properties of Na3OsO5

AbstractNa3OsO5 has been synthesized by solid state reaction of Na2O2 and osmium powder under a flow of oxygen. Na3OsO5 crystallizes trigonal in space group P3121 with a = 5.5529(2) Å, c = 13.4726(8) Å, V = 359.77(3) Å3, Z = 3, 958 independent reflections, R1 = 1,97 %, wR2 = 4.27 %. The crystal structure consists of OsO5 trigonal bipyramids separated by sodium cations, and is isostructural to Na3ReO5. The osmium atoms adopt a packing analogous to ccp, where Na occupies the octahedral and tetrahedral vacancies, so the cation structure is derived from the Li3Bi structure type. Features of the synthesis of Na3OsO5 and related osmates are discussed. Na3OsO5 shows paramagnetic behaviour down to 25 K where it orders antiferromagnetically. Below 7 K a weak ferromagnetic contribution is detected. The paramagnetic region of the 5d1 system has been successfully described in the temperature range 50 – 330 K accounting for strong spin‐orbit coupling, ligand‐field potential of D3h symmetry (corresponding to the trigonal bipyramidal $\rm OsO^{3-}_{5}$ unit), and cooperative magnetic effects (molecular field parameter λMF).

A computer simulation study of OH defects in Mg2SiO4 and Mg2GeO4 spinels

Classical atomistic simulation techniques have been used to investigate the energies of hydrogen defects in Mg2SiO4 and Mg2GeO4 spinels. Ringwoodite (γ-Mg2SiO4) is considered to be the most abundant mineral in the lower part of the transition zone and can incorporate large amounts of water in the form of hydroxyls, whereas the germanate spinel (γ-Mg2GeO4) corresponds to a low-pressure structural analogue for ringwoodite. The calculated defect energies indicate that the most favourable mechanisms for hydrogen incorporation are coupled either with the reduction of ferric iron or with the creation of tetrahedral vacancies. Hydrogen will go preferentially into tetrahedral vacancies, eventually leading to the formation of the hydrogarnet defect, before associating with other negatively charged point defects. The presence of isolated hydroxyls is not expected. The same trend is observed for germanate, and thus γ-Mg2GeO4 could be used as a low-pressure analogue for ringwoodite in studies of water-related defects and their effect on physical properties.

Atom relaxations around hydrogen defects in lanthanum hydride

Atom relaxations around empty octahedral and tetrahedral metal interstices in cubic lanthanum hydride LaH 2+ x have been studied experimentally and by ab initio theory. In-situ neutron powder diffraction data on the deuteride along the pressure–composition isotherm at 573 K reveal deuterium atom displacements of up to 0.6 Å within octahedral interstices. The displacements are directed from the centres towards the faces of the metal octahedra and are largest at deuterium contents of x ∼ 0 , i.e. in the presence of nearly empty octahedral and nearly filled tetrahedral interstices. As the deuterium content is increased the displacements decrease sharply and then increase slowly. Ab initio calculations on nearly stoichiometric LaH 3 (La 32H 95) confirm energetically favoured atom relaxations around empty hydrogen sites. The relaxations around tetrahedral vacancies are considerably larger than those around octahedral vacancies. Deuterium atoms in octahedral interstices, for example, relax by up to 0.8 Å which compares well with experiment. Empty metal tetrahedra contract by 0.06 Å while empty octahedra expand by 0.02 Å. Due to the strong relaxations around tetrahedral vacancies, formation of tetrahedral vacancies in LaH 3 is less favourable by just about 4 kJ mol −1 than formation of octahedral vacancies.

A New Family of Isostructural Titanates, MLi2Ti6O14 (M: Sr, Ba, Pb).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Characterization of dipole defects in MgAl 2 O 4 spinel

Dipole defects in gamma-irradiated and thermal treated MgAl2O4 samples have been studied through thermally stimulated depolarisation currents(TSDC) technique and computer modelling methods. The presence of TSDC bands varies from sample to sample and some crystals do not present any band. The origin of these bands has been investigated in several different samples. In the spectra of spinels showing TSDC peaks, three bands at 130K, 160K and 320K are observed. The peaks at 130K and 160K have been attributed to dipole defects. After 1200kGy of gamma irradiation the broad band at 320K dislocates to 290K and increases ten times. Pulsed thermal treatments between 350K and 470K produce a progressive reduction of the peak area and a shift in the peak position back to 320K. A detailed analysis of the curve indicates the possibility of a superposition of peaks. Gamma irradiation restores the 320K TSDC peak. Taking into account optical absorption(OA) and electron paramagnetic resonance(EPR) results, the thermal reduction of the 320K TSDC band was attributed to V-type centres as a result of hole trapping at tetrahedral and octahedral cation vacancies. Computer modelling methods, based on lattice energy and defect minimisation, were applied to identify dipole defects that occur in these crystals. The calculations were made in normal and inverse spinel structures, doped with Cr, Co, Mn and Fe in order to justify the presence of dipole bands. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In 3+ substitution effects and defect distribution in Li 0.25Mg 0.5Mn 0.1Fe 2.15−xIn xO 4 studied by positron annihilation and Mössbauer spectroscopy

The ferrite series Li 0.25Mg 0.5Mn 0.1Fe 2.15− x In x O 4 ( x = 0 , 0.3, 0.6, 0.9) have been studied through positron lifetime and Mössbauer spectroscopic techniques. The changes in positron lifetime parameters are attributed to positron trapping in tetrahedral and octahedral sites at the different stages of In 3+ ion substitution. Positrons initially get trapped in the vacancies in the octahedral sites in the undoped sample. With doping by In 3+ ions upto x = 0.3 , tetrahedral vacancies dominate as positron trapping sites. With further doping, some of the octahedral sites become vacancies and trap positrons. Incorporation of In 3+ ions in the place of Fe 3+ ions in the series has also introduced magnetic dilution in both the tetrahedral (A) and octahedral (B) sites. Knowledge of the variation in the magnetic properties of these ferrites with increase in concentration of In 3+ ions, as derived from the Mössbauer parameters, suggests a correlation of such a variation with the defect concentration in these samples.

A new family of isostructural titanates, MLi 2Ti 6O 14 (M = Sr, Ba, Pb)

The existence of isostructural titanates with a composition corresponding to the formula MLi 2Ti 6O 14 has been evidenced in the ternary oxide systems MO–Li 2O–TiO 2 (M = Sr, Ba, Pb). Single crystals of these compounds have been grown by the flux method using LiBO 2 as a high-temperature solvent. The structure, as determined by single crystal X-ray diffraction is orthorhombic (space group Cmca (no. 64), Z = 8) with (i) a = 16.570(5) Å, b = 11.150(2) Å, c = 11.458(2) Å, V = 2116.9(8) Å 3, R 1 = 0.039, w R 2 = 0.087 for SrLi 2Ti 6O 14; (ii) a = 16.577(1) Å, b = 11.273(1) Å, c = 11.581(1) Å, V = 2164.2(5) Å 3, R 1 = 0.018, w R 2 = 0.047 for BaLi 2Ti 6O 14 and (iii) a = 16.649(1) Å, b = 11.146(1) Å, c = 11.494(1) Å, V = 2132.9(5) Å 3, R 1 = 0.033, w R 2 = 0.078 for PbLi 2Ti 6O 14. A three-dimensional framework of edge and corner sharing TiO 6 octahedra sets up the crystal structure. Lithium occupies the tetrahedral vacancies, while M atoms are located at 11-oxygen coordinated sites.

Structure and electrical properties of CdIn2O4 thin films sputtered at elevated substrate temperatures

A Hall mobility as high as 39.4 cm2 V−1 s−1 was obtained for cadmium indate (CdIn2O4) thin films deposited by rf reactive sputtering from a Cd–In alloy target. The structural and electrical properties for both as-sputtered and after-annealed thin films were studied as a function of the substrate temperatures. The results revealed that elevated substrate temperatures favour the formation of CdIn2O4 thin films with spinel phase. The electrical properties can be understood by a combination of the occupancy of In cations on the tetrahedral vacancies in the spinel unit cell at the elevated substrate temperatures and the formation of oxygen vacancies originating from off-stoichiometry. It can be inferred that the presence of secondary phases in the CdIn2O4 thin films deposited in an oxygen-deficient atmosphere is responsible for their electrical properties. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)