Anion Vacancy Concentration Research Articles

Charge and mass transfer involving hydrogen in MgO crystals thermochemically reduced at high temperatures

Optical-absorption measurements show that ${\mathrm{H}}^{\ensuremath{-}}$ ions are thermally much more stable than anion vacancies when thermochemically reduced MgO samples are annealed in a reducing atmosphere. At 1900 K all anion vacancies are annihilated, but the concentration of ${\mathrm{H}}^{\ensuremath{-}}$ ions remains unchanged. Results of electron irradiations on a crystal annealed at 1900 K demonstrate unambiguously that the infrared-absorption peaks previously attributed to ${\mathrm{H}}^{\ensuremath{-}}$ ions are indeed due to protons in anion sublattice sites. ${\mathrm{H}}^{\ensuremath{-}}$ concentrations can be obtained from the infrared absorbance of the ${\mathrm{H}}^{\ensuremath{-}}$ ions. The lifetime of the 2.3-eV $F$-center phosphorescence is shown to be affected by the relative concentrations of ${\mathrm{H}}^{\ensuremath{-}}$ ions to anion vacancy. Decreasing the anion-vacancy concentration by thermal annealing, thereby increasing the relative concentration of ${\mathrm{H}}^{\ensuremath{-}}$ ions to anion vacancies, leads to an enhancement of the lifetime of the phosphorescence.

Kinetics of cation site exchange in mixed garnets

The redistribution rate of cations between the a- and d -sites in quasibinary mixed yttrium—iron gallium and—iron aluminum garnet single crystals has been investigated for temperatures between 773 and 1523 K. The frozen-in cation distributions of quenched and of slow cooled samples were derived from magnetization measurements and are described by a phenomenological kinetic equation. The activation energies derived for the thermally activated process range from 1.3 to 4eV and are related to the anion vacancy concentration of the crystals. The zero defect intrinsic a-dbarrier height is 4.5 eV as inferred from the observed linear relation between the frequency factor and the activation energy of the Arrhenius equation. This empirical relation, known as the compensation law, has been further applied to interpret reported intra-sublattice site exchange effects of cations on the c-sites and cation self-diffusion data of YIG. The inferred c-c barrier height of 5.5 eV agrees within the error limits of ± 0.1eV with the enthalpy of formation, 5.4 eV, and the enthalpy of solution, 5.3 eV, derived with equilibrium models based on the ionic fluid (Temkin) model and using experimental results of the kinetics, the cation distribution and the solubility of garnets. Apparently the c-c bonds form the strongest binding scaffold of the garnet structure, which is manifested by a relation to the melting point. While premelting around the anion vacancies assisted by their thermal fluctuations reduces the effective barrier height for cation migration, the apparently uneffected melting point of the compound itself is reflected by the compensation law.

Structural Chemistry of Co-Mo-Alumnina Catalysts

Abstract The structural chemistry of Co-Mo-alumina hydrodesulfurization catalysts has been critically reviewed in this article. The location and nature of cobalt in the sulfided catalysts has been discussed. Small MoS2, crystallites (10 to 30 Å) that occur on the surface of a sulfided sample are unstable due to the high, net negative charge on the sulfur ions on their edges. They incorporate cations like Co2+ at the edges of the MoS2 sheet (not intercalated between the sheets) and attain stability. It is shown that for the range of crystallito sizes of MoS, in these systems, a maximum of one Co ion can be incorporated per two Mo ions in this manner. Cobalt thus lends structural stability to the MoS2, crystallites and suppresses the excessive formation of anion vacancies at crystallite edges. Molybdenum sulfide crystallites on sulfided Mo-A12O3, lacking this stabilizing influence of cobalt, lose excess sulfur, leading to the creation of a larger concentration of multiple anion vacancies at their edges. The presence of these strongly acidic multiple vacancies, however, leads to a lower structural stability of these crystallites. Moreover, these strongly acidic sites are also deactivated faster in the presence of unsaturated molecules (like diolefins, olefins, etc.), accounting for the fact that the superiority of Co-Mo-Al2O3 over Mo-Al2O3 manifests itself mainly in the steady-state activity [61–63]. The advantages of this model over those currently in vogue are the following: (1) It offers an explanation for the rather high cobalt concentration [Co/(Co + Mo)=0.31 necessary for optimum catalytic activity, (2) it accounts for the established accessibility of cobalt in the Co-MoS2 phase to probe moledules [40], and (3) the stabilization of the small MoS2 crystallites in sulfided Co-Mo-Al2O3 compared to Mo-Al2O3 with the consequent lower loss of sulfur ions from the former in flowing hydrogen (Table 3 and Ref. 28). The migration and sintering of MoS2 crystallites in sulfided Mo-2O3 but not co-Mo-Al2O3, observed by Okamoto et al. [21], is also in accord with the above picture of stabilization of small MoS2 crstallites by cobalt.

A study of the anion mobility in mixed fluorides with the tysonite structure

From the studies which have been carried out it follows that in Berthollide solid solutions with the tysonite structure there is intensive diffusion of fluoride ions in the vacancies, and this is responsible for the electrical conductivity of the crystals. It follows from an analysis of the NMR data obtained that the fluorine vacancies are produced in the positions corresponding to fluorine atoms of low mobility, confirming structural studies which indicate the preferential formation of vacancies in the FIII positions. At low temperatures, the movement of the highly mobile FI fluorine atoms shows practically no association with the low-mobility FIII positions. Thus in spite of the high concentration of anion vacancies in the specimens studied, the mobility of fluorine at low temperatures does not increase, compared with the ideal tysonite structure. It follows from the temperature dependence of the19F NMR spectra, however, that the presence of vacancies increases the mobility in the subsystems of fluorine atoms with low mobility and increases the exchange between the fluorine atoms with high and low mobilities.

On the “anomalous” curvature of the diffusion plot of bromine in AgBr

AbstractThe excessive increase of cation Frenkel defects in AgBr near the melting point should supress the number of anion vacancies through the mass action law. Therefore a downward curvature in the bromine diffusion plot would be expected whereas the experiments show a strong upward curvature. It is shown that this “anomaly” is a consequence of the usual assumption that the Schottky‐Gibbs formation energy g decreases linearly with temperature. Furthermore it is shown that g has to decrease (with temperature) faster than linearly in such a way that the term (g/2) — g exhibits an excessive decrease with temperature. Therefore the concentration of anion vacancies shows an excessive increase nea the melting poing and hence the bromine diffusion plot has to show an upward curvature. Another contribution — but a smaller one — to this curvature is the non‐linear decrease of the migration Gibbs energy of a free anion vacancy with the temperature.

Hydrodesulfurization of thiophene, benzothiophene, dibenzothiophene, and related compounds catalyzed by sulfided CoO$z.sbnd;MoO3/$gamma;-Al2O3: Low-pressure reactivity studies

Hydrodesulfurization experiments were carried out with a sulfided CoOMoO3γ-Al2O3 catalyst in a pulse microreactor operated at atmospheric pressure and temperatures of 350 to 450 °C. The reactants were hydrogen and pure sulfur-containing compounds (or pairs of compounds), including thiophene, benzothiophene, dibenzothiophene, several of their hydrogenated derivatives, and various methyl-substituted benzothiophenes and dibenzothiophenes. The aromatic compounds appeared to react with hydrogen by simple sulfur extrusion; for example, dibenzothiophene gave H2S + biphenyl in the absence of side products. The reactivities of thiophene, benzothiophene, and dibenzothiophene were roughly the same. Each hydrogenated compound (e.g., tetrahydrothiophene) was more reactive than the corresponding aromatic compound (e.g., thiophene). Methyl substituents on benzothiophene had almost no effect on reactivity, whereas methyl substituents on dibenzothiophene located at a distance from the S atom slightly increased the reactivity, and those in the 4-position or in the 4- and 6-positions significantly decreased the reactivity. In contrast to the observation of a near lack of dependence of low-pressure reactivity on the number of rings in the reactant, the literature shows that at high pressures the reactivity decreases with an increased number of rings. The pressure dependence of the structure-reactivity pattern is suggested to be an indication of relatively less surface coverage by the intrinsically more reactive compounds (e.g., thiophene) at low pressures but not at high pressures. The relative reactivities are also suggested to be influenced by differences in the structures of the catalyst at low and high hydrogen partial pressures, which may be related to the concentrations of surface anion vacancies and the nature of the adsorbed intermediates.

Effect of Intentional Impurities, Aging, Particle Size and Pre‐Irradiation on the Thermal Decomposition of Hydrazinium Diperchlorate

AbstractThe effect of dopants, aging, particle size and pre‐irradiation on the thermal decomposition of hydrazinium diperchlorate (HP‐2) has been investigated. Data on the uptake of N2H6S04 indicate that these impurities are distributed homogenously in HP‐2. Anion vacancies have been found to sensitize the decomposition while cation vacancies desensitize. The mechanism of the decomposition has been discussed in terms of the migration of ionic species and electron transfer from CIO4− ion to N2H62+ ion.The decomposition is found to desensitize with age. The aging effect is attributed to a decrease in anion vacancy concentration with age. Particle size does not seem to have any effect on the decomposition of HP‐2. Pre‐irradiation is found to affect the decomposition appreciably only when HP‐2 is irradiated for longer durations (> 1 hr), the effect being observed only during the later stages of the decomposition (01 > 0.3).

The reduction of MoO3at low temperatures

Abstract The reduction of MoO3 in H2 was studied in situ in a high-voltage electron microscope. A new type of defect close to the surface and parallel to the [101] directions was observed, which nucleated abruptly at a temperature below 80°C, and which disappeared on exposure to air. It is proposed that a high concentration of anion vacancies is generated at the surface as a result of the chemical reduction, at a temperature at which diffusion is relatively slow. This leads to the nucleation of a collapsed ribbon of vacancies bounded by a partial edge dislocation, Burgers vector type ½ [101]. The model for the defect is consistent with image-contrast experiments. The conditions for nucleation of the vacancy discs are discussed, and estimates are made of the activation energies of migration of anion vacancies in MoO3.

Effects of Ultraviolet Irradiation on KBr: Gd

AbstractThe results of UV irradiation on Gd3+ doped KBr single crystals were investigated. Thermoluminescence (TL) glow curves, excitation spectra, and optical absorption were measured in comparison with pure and Sr2+ doped KBr crystals. The concentration of anion vacancies was found to be markedly enhanced by the presence of impurity ions. TL excitation spectra of Gd3+ doped samples had maxima at the α band and at the 212 nm band, which is attributed to a localized exciton state perturbed by a Gd3+ ion. Results suggest that defects responsible for the TL are created by an excitonic process.

The oxidation behavior of a Zr-0.5Y alloy

A Zr-0.5 Y alloy was found to oxidize about 6 times faster than pure zirconium over the temperature range of 400 to 565°C. The activation energies were nearly identical (∼32 kcal/mole). The activation energies correspond to grain boundary diffusion of oxygen through the scale. The higher oxidation rate of the alloy was attributed to a higher anion vacancy concentration and the assumption that diffusion sites in the lattice and boundaries were in local equilibrium. Measurements on yttria-doped zirconia showed that ionic conductivity was increased markedly by yttrium and extended over a wide range of oxygen pressure. The defect structure of the doped oxide was changed to one of oxygen vacancies, even at the high end of the oxygen pressure range, 10−8 to 0.2 atm, over which pure ziconia contains oxygen interstitials. The doped oxide was found to be extrinsic over the entire range of oxygen pressure and, although ionic conductivity predominated, electronic conductivity was still appreciable. The electronic conductivity, however, was still sufficiently high so that electron transport was not rate-controlling in the predominantly ionic-conducting scale.

Effect of Non-Stoichiometry in Ni-Ferrites on Electrical Conduction

Five single crystals with nearly same composition but different vacancy concentration between -0.005 and +0.035 in formular unit are grown by the Bridgman method. Resistivity in the frequency range between 100 Hz and 100 MHz and Seebeck coefficient are measured in temperatures ranging from 78 to 300 K. The electrically obtained concentration of Fe 2+ ions is in good agreement with that obtained by chemical analysis. The activation energy of an electron bound to a Fe 2+ ion on B sites is of the order of 10 -2 eV and increases with increasing vacancy concentration. The polaron binding energy is of the same order as the electron binding energy, and corresponds to that of small-polaron conductors. This energy is independent of cation vacancy concentration between 4.7×10 20 and 11×10 20 per unit volume but decreases 13% with the formation of anion vacancy concentration of 1.1×10 20 .

Studies of molybdena-alumina catalysts: IV. Rates and stoichiometry of sulfidation

Molybdena-alumina catalysts were sulfided with H 2 S H 2 blends either directly from the calcined (oxidized) state or after prereduction with H 2. The course of the reaction was followed gravimetrically; sulfur analyses at the end of a run permitted material balance calculations to be made on the end state of the catalyst. Sulfided and reduced catalysts were additionally characterized by NH 3 adsorption and D 2 exchange. Catalyst sulfiding occurred readily above 300 °C. Extent of sulfiding increased with temperature. However, a limiting catalyst sulfur content was obtained at a given temperature; neither increase in H 2S partial pressure nor time much affected the sulfur level. The predominant reaction was exchange of oxygen associated with the molybdena (reactive oxygen) for sulfur, with formation of water. At higher temperatures, some additional reactive oxygen was lost due to a reductive reaction (also forming water), which presumably created anion vacancies. Prereduced catalysts sulfided to a lesser extent, even though some sulfur apparently added to anion vacancies present after the prereduction. Ammonia adsorption on sulfided and reduced catalysts showed a correlation with anion vacancy concentration. Exchange measurements with D 2 revealed that the sulfided catalyst irreversibly retained appreciable hydrogen, probably as −SH groups on the surface; the magnitude of the retained H was far greater than that found for reduced catalysts. The Al 2O 3 portion of the catalyst appeared to contain less hydrogen than that characteristic of the pure Al 2O 3 base. A model of the catalyst surface, consisting of one-dimensional, chain-like groupings of MoO 2 over the Al 2O 3 substrate, is proposed to explain the results.

Crystallite growth in magnesium oxide under applied electric fields

An electrolysis cell is described for use in conjunction with a high-temperature X-ray furnace, which permits X-ray diffraction studies to be made of solids during the course of their high temperature electrolysis. The technique is applied to an investigation of the effect of electric fields on crystallite growth in MgO at 600°C in air, argon and reducing atmospheres. In all cases crystallite growth is greatest at the positive electrode face, suggesting that the charge carriers are anions. The efficiency of the electrolysis process depends both on the concentration of charge carriers and on the concentration of anion vacancies by which the charge carriers migrate. The defect concentration is increased under reducing conditions. In MgO samples containing a high concentration of protons, the conduction is protonic, with the greatest crystallite growth occurring at the cathode due to the catalytic action of the water vapour formed in that region by recombination of protons with hydroxyl ions.

Isothermal grain growth and sintering in pure ThO 2 and ThO 2-CaO compositions

Isothermal sintering and grain growth was studied in ThO 2-CaO compositions with 0, 2.31, 4.25 and 8.8 mole% CaO at temperatures of 1300, 1400, 1500, 1550 and 1600 °C for 5, 10, 20, 30, 60 and 120 min and the data were correlated against Coble's model of intermediate stage sintering kinetics. Diffusion coefficients for the respective compositions were calculated as 1.3 × 10 −1 exp (−93 000/ RT), 4.02 × 10 −6 exp (− 56 000/ RT), 4.35 × 10 −7 exp (− 47 000/ RT) and 3.43 × 10 −7 exp (− 45 000/ RT) cm 2/ sec. The decrease in the activation energy with increasing percentage of CaO is qualitatively explained in terms of the effect of anion vacancy concentration on cation migration

Influence of V<sub>2</sub>O<sub>5</sub> Addition on the Magnetic and Electrical Properties of Iron-Deficient Ni-Zn Ferrite

The influence of V2O5 addition on the magnetic and electrical properties of iron deficient ferrites with composition: Ni0.65Zn0.35Fe1.96O4+xV2O5 have been investigated, where values of x (in mol unit) are 0, 0.002, 0.004, 0.006, 0.008 and 0.01. The sintered density, saturation magnetization, Curie temperature of the samples increase up to V2O5 (x=0.004) and then decrease. The addition of V2O5 to the composition of ferrite increases the anion vacancy concentration, prohibit the discontinuous grain growth, which in turn enhances the density. The increment of saturation magnetization is due to the enhancement of resultant sublattice magnetic moment and density. Upto V2O5 (x=0.004), V2O5 goes into solid solution with the ferrite matrix and increases the Curie temperature. This is probably due to ions occupy some octahedral sites (B-sites) of the ferrite lattice by V5+ , as a result some Fe3+ ions may come out from octahedral to tetrahedral sites (A-sites) thereby increasing A-B interaction and hence the Curie temperature. The dc resistivity of all the samples increases with the increment of V2O5 in the compound. DOI: 10.3329/jbas.v33i2.4097 Journal of Bangladesh Academy of Sciences, Vol. 33, No. 2, 145-149, 2009

Self-diffusion of oxygen in magnetite. Effect of anion vacancy concentration and cation distribution

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSelf-diffusion of oxygen in magnetite. Effect of anion vacancy concentration and cation distributionJames E. Castle and P. L. SurmanCite this: J. Phys. Chem. 1969, 73, 3, 632–634Publication Date (Print):March 1, 1969Publication History Published online1 May 2002Published inissue 1 March 1969https://doi.org/10.1021/j100723a025RIGHTS & PERMISSIONSArticle Views433Altmetric-Citations57LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (328 KB) Get e-Alerts

Ionic conductivity of potassium iodide crystals

Ionic conductivity of pure and doped KI is measured in the temperature range 200 to 700°c. A large contribution to the conductivity by the anion vacancies is observed in pure KI at high temperatures. The solubility of Ba2+ in KI is much higher than that of Cd2+, Pb2+ and Ca2+. The behaviour of KI doped with Ba2+ is similar to the behaviour of KCl and NaCl doped with divalent cation impurities. The solubility of CO32- in KI is also good. Crystals doped with CO32- show an increase in conductivity at high temperatures owing to the increase in anion vacancy concentration. At lower temperatures the conductivity of KI is suppressed by the CO32- impurity. The values obtained for various energies of activation in KI crystals are Ec = (0·72±0·02) ev for the migration of a cation vacancy, EA = (1·50±0·05) ev for the migration of an anion vacancy, Ws = (1·6±0·1) ev for the formation of a separated pair of Schottky vacancies and Wc similar 0·26 ev for the association of a cation vacancy with Ba2+ impurity.

Structural vacancies and solid solution decay in ferrospinels

The formation of vacancies in nonstoichiometric ferrospinels and their effect, upon solid solution stability are considered. On the basis of a solution model including vacancies of both signs, the interdependency of cation and anion vacancy concentrations, and of degree of inversion and magnetite content in solutions of Mg 1− a Fe 2+ a O 4 at a fixed temperature are obtained. It is shown that vacancy concentration can be considered as a parameter determining the structure stability. Exceeding a critical value of this parameter in a given solution leads to instability. Within the temperature range considered, this critical vacancy concentration is not temperature-dependent. The estimated interdependencies are verified by X-ray, chemical and magnetic data.

Czochralski-growth of calcium fluorophosphate

Large single crystals of calcium fluorophosphate(apatite) have been grown by the Czochralski method. Crystals grown along major crystallographic axes exhibit etch pit shape and crystal morphology consistent with the hexagonal unit cell. The phase diagram for the system has been re-examined and crystals were grown at several off-stoichiometric compositions. Chemical analysis of these crystals indicated fluoride vacancy concentrations as large as 10 at %. This is consistent with the data obtained from lattice parameter and density measurements. The observation of the fluorescence characteristics of these undoped crystals is consistent with color center fluorescence associated with fluoride vacancies. Deliberate replacement of fluoride with oxygen indicated that anion vacancy concentrations in the fluoride site as high as 50% could be tolerated by the structure. Measurement of physical constants such as dielectric constant, thermal expansion and refractive index indicates remarkably little anisotropy in the major crystallographic directions.

Zinc-Ferriteに関する研究 (第4報)

In order to clarify the dissolution mechanism of zinc-ferrite, the effects of the cooling rate, Fe2O3/ZnO ratio, and the formation atmosphere on the dissolution rate were investigated.In the present experiments, the following results were obtained.(1) The zinc-ferrite formed at 1000°C has the same intrinsic dissolution rate k (g·min-1·m-2) irrespective of the cooling rate. The dissolution rate of that formed above 1100°C, depends on the coolingrate and, if cooled rapidly, the rate increases considerably, whereas, if cooled slowly, the rate remainsto. be much the same as thas of 1000°C.(2) If excess Fe2O3 is present in zinc-ferrite in solid solution, the intrinsic dissolution ratedecreases and the more Fe2O3 in zinc-ferrite, the smaller is the dissolution rate. The rate increases againif Fe2O3 is precipitated during slow cooling.(3) If the zinc-ferrite formed at 1200°C is heated in vacuum at 1000°C, the intrinsic dissolutionrate increases considerably, while, if heated in air at 1000°C, the rate becomes almost the same value asthat of the zinc-ferrite originally formed at 1000°C.From these results, it can be concluded that the zinc-ferrite of 02-deficient composition dissolvesin dilute sulfuric acid more rapidly than that of stoichiometric composition, and that therefore the dissolutionof zinc-ferrite-is initiated from the anion vacancy sites and the rate is controlled by the concentrationof anion vacancies.